Homosexuality is romantic attraction, sexual attraction or sexual behavior between members of the same sex or gender. As a sexual orientation, homosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to people of the same sex. It "also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share those attractions."  
Along with bisexuality and heterosexuality, homosexuality is one of the three main categories of sexual orientation within the heterosexual–homosexual continuum.  Scientists do not know exactly what determines an individual's sexual orientation, but they believe that it is caused by a complex interplay of genetic, hormonal, and environmental influences,    and do not view it as a choice.    They favor biologically-based theories,  which point to genetic factors, the early uterine environment, both, or the inclusion of genetic and social factors.   There is no substantive evidence which suggests parenting or early childhood experiences play a role when it comes to sexual orientation.  While some people believe that homosexual activity is unnatural,  scientific research has shown that homosexuality is a normal and natural variation in human sexuality and is not in and of itself a source of negative psychological effects.   There is insufficient evidence to support the use of psychological interventions to change sexual orientation. 
The most common terms for homosexual people are lesbian for females and gay for males, though gay is also used to refer generally to both homosexual males and females. The number of people who identify as gay or lesbian and the proportion of people who have same-sex sexual experiences are difficult for researchers to estimate reliably for a variety of reasons, including many gay or lesbian people not openly identifying as such due to homophobia and heterosexist discrimination. Homosexual behavior has also been documented in many non-human animal species.   
Many gay and lesbian people are in committed same-sex relationships, though only recently have census forms and political conditions facilitated their visibility and enumeration.  These relationships are equivalent to heterosexual relationships in essential psychological respects.  Homosexual relationships and acts have been admired, as well as condemned, throughout recorded history, depending on the form they took and the culture in which they occurred.  Since the end of the 19th century, there has been a global movement towards increased visibility, recognition, and legal rights for gay people, including the rights to marriage and civil unions, adoption and parenting, employment, military service, equal access to health care, and the introduction of anti-bullying legislation to protect gay minors.
The word homosexual is a Greek and Latin hybrid, with the first element derived from Greek ὁμός homos , "same" (not related to the Latin homo , "man", as in Homo sapiens ), thus connoting sexual acts and affections between members of the same sex, including lesbianism. The first known appearance of homosexual in print is found in an 1869 German pamphlet by the Austrian-born novelist Karl-Maria Kertbeny, published anonymously,  arguing against a Prussian anti- sodomy law.   In 1886, the psychiatrist Richard von Krafft-Ebing used the terms homosexual and heterosexual in his book Psychopathia Sexualis . Krafft-Ebing's book was so popular among both laymen and doctors that the terms "heterosexual" and "homosexual" became the most widely accepted terms for sexual orientation.   As such, the current use of the term has its roots in the broader 19th-century tradition of personality taxonomy.
Many modern style guides in the U.S. recommend against using homosexual as a noun, instead using gay man or lesbian .  Similarly, some recommend completely avoiding usage of homosexual as it has a negative, clinical history and because the word only refers to one's sexual behavior (as opposed to romantic feelings) and thus it has a negative connotation.  Gay and lesbian are the most common alternatives. The first letters are frequently combined to create the initialism LGBT (sometimes written as GLBT), in which B and T refer to bisexual and transgender people.
Gay especially refers to male homosexuality,  but may be used in a broader sense to refer to all LGBT people. In the context of sexuality, lesbian refers only to female homosexuality. The word lesbian is derived from the name of the Greek island Lesbos, where the poet Sappho wrote largely about her emotional relationships with young women. 
Although early writers also used the adjective homosexual to refer to any single-sex context (such as an all-girls school), today the term is used exclusively in reference to sexual attraction, activity, and orientation. The term homosocial is now used to describe single-sex contexts that are not specifically sexual. There is also a word referring to same-sex love, homophilia .
Some synonyms for same-sex attraction or sexual activity include men who have sex with men or MSM (used in the medical community when specifically discussing sexual activity) and homoerotic (referring to works of art).  Pejorative terms in English include queer , faggot , fairy , poof , and homo .   Beginning in the 1990s, some of these have been reclaimed as positive words by gay men and lesbians, as in the usage of queer studies, queer theory, and even the popular American television program Queer Eye for the Straight Guy .  The word homo occurs in many other languages without the pejorative connotations it has in English.  As with ethnic slurs and racial slurs, the misuse of these terms can still be highly offensive. The range of acceptable use for these terms depends on the context and speaker.  Conversely, gay , a word originally embraced by homosexual men and women as a positive, affirmative term (as in gay liberation and gay rights),  has come into widespread pejorative use among young people. 
Societal attitudes towards same-sex relationships have varied over time and place, from expecting all males to engage in same-sex relationships, to casual integration, through acceptance, to seeing the practice as a minor sin, repressing it through law enforcement and judicial mechanisms, and to proscribing it under penalty of death.
In a detailed compilation of historical and ethnographic materials of Preindustrial Cultures, "strong disapproval of homosexuality was reported for 41% of 42 cultures; it was accepted or ignored by 21%, and 12% reported no such concept. Of 70 ethnographies, 59% reported homosexuality absent or rare in frequency and 41% reported it present or not uncommon."
In cultures influenced by Abrahamic religions, the law and the church established sodomy as a transgression against divine law or a crime against nature. The condemnation of anal sex between males, however, predates Christian belief. It was frequent in ancient Greece; "unnatural" can be traced back to Plato.
Many historical figures, including Socrates, Lord Byron, Edward II, and Hadrian, have had terms such as gay or bisexual applied to them; some scholars, such as Michel Foucault, have regarded this as risking the anachronistic introduction of a contemporary construction of sexuality foreign to their times, though others challenge this.
In social science, there has been a dispute between "essentialist" and "constructionist" views of homosexuality. The debate divides those who believe that terms such as "gay" and "straight" refer to objective, culturally invariant properties of persons from those who believe that the experiences they name are artifacts of unique cultural and social processes. "Essentialists" typically believe that sexual preferences are determined by biological forces, while "constructionists" assume that sexual desires are learned. The philosopher of science Michael Ruse has stated that the social constructionist approach, which is influenced by Foucault, is based on a selective reading of the historical record that confuses the existence of homosexual people with the way in which they are labelled or treated. 
The first record of possible homosexual couple in history is commonly regarded as Khnumhotep and Niankhkhnum, an ancient Egyptian male couple, who lived around 2400 BCE. The pair are portrayed in a nose-kissing position, the most intimate pose in Egyptian art, surrounded by what appear to be their heirs. The anthropologists Stephen Murray and Will Roscoe reported that women in Lesotho engaged in socially sanctioned "long term, erotic relationships" called motsoalle. The anthropologist E. E. Evans-Pritchard also recorded that male Azande warriors in the northern Congo routinely took on young male lovers between the ages of twelve and twenty, who helped with household tasks and participated in intercrural sex with their older husbands. 
Among indigenous peoples of the Americas prior to European colonization, a common form of same-sex sexuality centered around the figure of the Two-Spirit individual. Typically, this individual was recognized early in life, given a choice by the parents to follow the path and, if the child accepted the role, raised in the appropriate manner, learning the customs of the gender it had chosen. Two-Spirit individuals were commonly shamans and were revered as having powers beyond those of ordinary shamans. Their sexual life was with the ordinary tribe members of the same sex.
Homosexual and transgender individuals were also common among other pre- conquest civilizations in Latin America, such as the Aztecs, Mayas, Quechuas, Moches, Zapotecs, and the Tupinambá of Brazil.  
The Spanish conquerors were horrified to discover sodomy openly practiced among native peoples, and attempted to crush it out by subjecting the berdaches (as the Spanish called them) under their rule to severe penalties, including public execution, burning and being torn to pieces by dogs. 
In 1998, the state of Hawaii passed a constitutional amendment against gay marriage.  In 2013 a ruling, by the state attorney general on this amendment, allowed the government to pass a statute legalizing gay marriage. 
In East Asia, same-sex love has been referred to since the earliest recorded history.
Homosexuality in China, known as the passions of the cut peach and various other euphemisms, has been recorded since approximately 600 BCE. Homosexuality was mentioned in many famous works of Chinese literature. The instances of same-sex affection and sexual interactions described in the classical novel Dream of the Red Chamber seem as familiar to observers in the present as do equivalent stories of romances between heterosexual people during the same period. Confucianism, being primarily a social and political philosophy, focused little on sexuality, whether homosexual or heterosexual. Ming Dynasty literature, such as Bian Er Chai (弁而釵/弁而钗), portray homosexual relationships between men as more enjoyable and more "harmonious" than heterosexual relationships. Writings from the Liu Song Dynasty by Wang Shunu claimed that homosexuality was as common as heterosexuality in the late 3rd century. 
Opposition to homosexuality in China originates in the medieval Tang Dynasty (618–907), attributed to the rising influence of Christian and Islamic values, but did not become fully established until the Westernization efforts of the late Qing Dynasty and the Republic of China.
In June 2014, 20,000 supporters demonstrated in favor of gay rights.
On October 29, 2014 Singapore High Court dismissed a constitutional challenge against a statute against sodomy. The statute provides a sentence of up to 2 years in jail. 
The Laws of Manu mentions a "third sex", members of which may engage in nontraditional gender expression and homosexual activities.
In regard to male homosexuality, such documents depict a world in which relationships with women and relationships with youths were the essential foundation of a normal man's love life. Same-sex relationships were a social institution variously constructed over time and from one city to another. The formal practice, an erotic yet often restrained relationship between a free adult male and a free adolescent, was valued for its pedagogic benefits and as a means of population control, though occasionally blamed for causing disorder. Plato praised its benefits in his early writings but in his late works proposed its prohibition. Aristotle, in the Politics , dismissed Plato's ideas about abolishing homosexuality (2.4); he explains that barbarians like the Celts accorded it a special honor (2.6.6), while the Cretans used it to regulate the population (2.7.5).
Little is known of female homosexuality in antiquity. Sappho, born on the island of Lesbos, was included by later Greeks in the canonical list of nine lyric poets. The adjectives deriving from her name and place of birth ( Sapphic and Lesbian) came to be applied to female homosexuality beginning in the 19th century.   Sappho's poetry centers on passion and love for various personages and both genders. The narrators of many of her poems speak of infatuations and love (sometimes requited, sometimes not) for various females, but descriptions of physical acts between women are few and subject to debate.
In Ancient Rome the young male body remained a focus of male sexual attention, but relationships were between older free men and slaves or freed youths who took the receptive role in sex. The Hellenophile emperor Hadrian is renowned for his relationship with Antinous, but the Christian emperor Theodosius I decreed a law on 6 August 390, condemning passive males to be burned at the stake. Justinian, towards the end of his reign, expanded the proscription to the active partner as well (in 558), warning that such conduct can lead to the destruction of cities through the "wrath of God". Notwithstanding these regulations, taxes on brothels of boys available for homosexual sex continued to be collected until the end of the reign of Anastasius I in 518.
During the Renaissance, wealthy cities in northern Italy — Florence and Venice in particular — were renowned for their widespread practice of same-sex love, engaged in by a considerable part of the male population and constructed along the classical pattern of Greece and Rome. But even as many of the male population were engaging in same-sex relationships, the authorities, under the aegis of the Officers of the Night court, were prosecuting, fining, and imprisoning a good portion of that population.
From the second half of the 13th century, death was the punishment for male homosexuality in most of Europe.  The relationships of socially prominent figures, such as King James I and the Duke of Buckingham, served to highlight the issue, including in anonymously authored street pamphlets: "The world is chang'd I know not how, For men Kiss Men, not Women now;...Of J. the First and Buckingham: He, true it is, his Wives Embraces fled, To slabber his lov'd Ganimede" ( Mundus Foppensis, or The Fop Display'd , 1691).
Love Letters Between a Certain Late Nobleman and the Famous Mr. Wilson was published in 1723 in England and was presumed by some modern scholars to be a novel. The 1749 edition of John Cleland's popular novel Fanny Hill includes a homosexual scene, but this was removed in its 1750 edition. Also in 1749, the earliest extended and serious defense of homosexuality in English, Ancient and Modern Pederasty Investigated and Exemplified , written by Thomas Cannon, was published, but was suppressed almost immediately. It includes the passage, "Unnatural Desire is a Contradiction in Terms; downright Nonsense. Desire is an amatory Impulse of the inmost human Parts." Around 1785 Jeremy Bentham wrote another defense, but this was not published until 1978.  Executions for sodomy continued in the Netherlands until 1803, and in England until 1835.
Between 1864 and 1880 Karl Heinrich Ulrichs published a series of twelve tracts, which he collectively titled Research on the Riddle of Man-Manly Love. In 1867, he became the first self-proclaimed homosexual person to speak out publicly in defense of homosexuality when he pleaded at the Congress of German Jurists in Munich for a resolution urging the repeal of anti-homosexual laws. Sexual Inversion by Havelock Ellis, published in 1896, challenged theories that homosexuality was abnormal, as well as stereotypes, and insisted on the ubiquity of homosexuality and its association with intellectual and artistic achievement.
Although medical texts like these (written partly in Latin to obscure the sexual details) were not widely read by the general public, they did lead to the rise of Magnus Hirschfeld's Scientific-Humanitarian Committee, which campaigned from 1897 to 1933 against anti-sodomy laws in Germany, as well as a much more informal, unpublicized movement among British intellectuals and writers, led by such figures as Edward Carpenter and John Addington Symonds. Beginning in 1894 with Homogenic Love , Socialist activist and poet Edward Carpenter wrote a string of pro-homosexual articles and pamphlets, and "came out" in 1916 in his book My Days and Dreams . In 1900, Elisar von Kupffer published an anthology of homosexual literature from antiquity to his own time, Lieblingminne und Freundesliebe in der Weltliteratur .
There are a handful of accounts by Arab travelers to Europe during the mid-1800s. Two of these travelers, Rifa'ah al-Tahtawi and Muhammad as-Saffar, show their surprise that the French sometimes deliberately mistranslated love poetry about a young boy, instead referring to a young female, to maintain their social norms and morals.
Israel is considered the most tolerant country in the Middle East and Asia to homosexuals with Tel Aviv being named "the gay capital of the Middle East", and is considered one of the most gay friendly cities in the world. The annual Pride Parade in support of homosexuality takes place in Tel Aviv. 
On the other hand, many governments in the Middle East often ignore, deny the existence of, or criminalize homosexuality. Homosexuality is illegal in almost all Muslim countries.  Same-sex intercourse officially carries the death penalty in several Muslim nations: Saudi Arabia, Iran, Mauritania, northern Nigeria, Sudan, and Yemen.  Iranian President Mahmoud Ahmadinejad, during his 2007 speech at Columbia University, asserted that there were no gay people in Iran. However, the probable reason is that they keep their sexuality a secret for fear of government sanction or rejection by their families.
In ancient Sumer, a set of priests known as gala worked in the temples of the goddess Inanna, where they performed elegies and lamentations.  : 285 Gala took female names, spoke in the eme-sal dialect, which was traditionally reserved for women, and appear to have engaged in homosexual intercourse.  The Sumerian sign for gala was a ligature of the signs for "penis" and "anus".  One Sumerian proverb reads: "When the gala wiped off his ass [he said], 'I must not arouse that which belongs to my mistress [i.e., Inanna].'"  In later Mesopotamian cultures, kurgarrū and assinnu were servants of the goddess Ishtar (Inanna's East Semitic equivalent), who dressed in female clothing and performed war dances in Ishtar's temples.  Several Akkadian proverbs seem to suggest that they may have also engaged in homosexual intercourse. 
In ancient Assyria, homosexuality was present and common; it was also not prohibited, condemned, nor looked upon as immoral or disordered. Some religious texts contain prayers for divine blessings on homosexual relationships. The Almanac of Incantations contained prayers favoring on an equal basis the love of a man for a woman, of a woman for a man, and of a man for man.
In Greater Iran, homosexuality and homoerotic expressions were tolerated in numerous public places, from monasteries and seminaries to taverns, military camps, bathhouses, and coffee houses. In the early Safavid dynasty (1501–1723), male houses of prostitution ( amrad khane ) were legally recognized and paid taxes.
Some scholars argue that there are examples of homosexual love in ancient literature, like in the Mesopotamian Epic of Gilgamesh as well as in the Biblical story of David and Jonathan. In the Epic of Gilgamesh, the relationship between the main protagonist Gilgamesh and the character Enkidu has been seen by some to be homosexual in nature.  Similarly, David's love for Jonathan is "greater than the love of women."
In many societies of Melanesia, especially in Papua New Guinea, same-sex relationships were an integral part of the culture until the middle of the last century. The Etoro and Marind-anim for example, viewed heterosexuality as sinful and celebrated homosexuality instead. In many traditional Melanesian cultures a prepubertal boy would be paired with an older adolescent who would become his mentor and who would "inseminate" him (orally, anally, or topically, depending on the tribe) over a number of years in order for the younger to also reach puberty. Many Melanesian societies, however, have become hostile towards same-sex relationships since the introduction of Christianity by European missionaries.
Sexuality and identity
Behavior and desire
The American Psychological Association, the American Psychiatric Association, and the National Association of Social Workers identify sexual orientation as "not merely a personal characteristic that can be defined in isolation. Rather, one's sexual orientation defines the universe of persons with whom one is likely to find the satisfying and fulfilling relationships": 
Sexual orientation is commonly discussed as a characteristic of the individual, like biological sex, gender identity, or age. This perspective is incomplete because sexual orientation is always defined in relational terms and necessarily involves relationships with other individuals. Sexual acts and romantic attractions are categorized as homosexual or heterosexual according to the biological sex of the individuals involved in them, relative to each other. Indeed, it is by acting—or desiring to act—with another person that individuals express their heterosexuality, homosexuality, or bisexuality. This includes actions as simple as holding hands with or kissing another person. Thus, sexual orientation is integrally linked to the intimate personal relationships that human beings form with others to meet their deeply felt needs for love, attachment, and intimacy. In addition to sexual behavior, these bonds encompass nonsexual physical affection between partners, shared goals and values, mutual support, and ongoing commitment. 
The Kinsey scale, also called the Heterosexual-Homosexual Rating Scale,  attempts to describe a person's sexual history or episodes of his or her sexual activity at a given time. It uses a scale from 0, meaning exclusively heterosexual, to 6, meaning exclusively homosexual. In both the Male and Female volumes of the Kinsey Reports, an additional grade, listed as "X", has been interpreted by scholars to indicate asexuality. 
In a 2004 study, the female subjects (both gay and straight women) became sexually aroused when they viewed heterosexual and lesbian erotic films. Among the male subjects, however, the straight men were turned on only by erotic films with women, the gay ones by those with men. The study's senior researcher said that women's sexual desire is less rigidly directed toward a particular sex, as compared with men's, and it is more changeable over time. 
Sexual orientation identity and sexual fluidity
Often, sexual orientation and sexual orientation identity are not distinguished, which can impact accurately assessing sexual identity and whether or not sexual orientation is able to change; sexual orientation identity can change throughout an individual's life, and may or may not align with biological sex, sexual behavior or actual sexual orientation.   While the Centre for Addiction and Mental Health and American Psychiatric Association state that sexual orientation is innate, continuous or fixed throughout their lives for some people, but is fluid or changes over time for others,   the American Psychological Association distinguishes between sexual orientation (an innate attraction) and sexual orientation identity (which may change at any point in a person's life). 
A 2012 study found that 2% of a sample of 2,560 adult participants reported a change of sexual orientation identity after a 10-year period. For men, a change occurred in 0.78% of those who had identified as heterosexual, 9.52% of homosexuals, and 47% of bisexuals. For women, a change occurred in 1.36% of heterosexuals, 63.6% of lesbians, and 64.7% of bisexuals. The researchers suggested that heterosexuality may be a more stable identity because of its normative status. 
A 2-year study by Lisa M. Diamond on a sample of 80 non-heterosexual female adolescents (age 16-23) reported that half of the participants had changed sexual-minority identities more than once, one third of them during the 2-year follow-up. Diamond concluded that "although sexual attractions appear fairly stable, sexual identities and behaviors are more fluid." 
People with a homosexual orientation can express their sexuality in a variety of ways, and may or may not express it in their behaviors.  Many have sexual relationships predominately with people of their own gender identity, though some have sexual relationships with those of the opposite gender, bisexual relationships, or none at all ( celibacy).  Survey data indicates that between 40% and 60% of gay men and between 45% and 80% of lesbians are currently involved in a romantic relationship.  Survey data also indicate that between 18% and 28% of gay couples and between 8% and 21% of lesbian couples in the U.S. have lived together ten or more years.  Studies have found same-sex and opposite-sex couples to be equivalent to each other in measures of satisfaction and commitment in relationships, that age and gender are more reliable than sexual orientation as a predictor of satisfaction and commitment to a relationship, and that people who are heterosexual or homosexual share comparable expectations and ideals with regard to romantic relationships.   
Coming out of the closet
Coming out ( of the closet ) is a phrase referring to one's disclosure of their sexual orientation or gender identity, and is described and experienced variously as a psychological process or journey.  Generally, coming out is described in three phases. The first phase is that of "knowing oneself", and the realization emerges that one is open to same-sex relations.  This is often described as an internal coming out. The second phase involves one's decision to come out to others, e.g. family, friends, or colleagues. The third phase more generally involves living openly as an LGBT person.  In the United States today, people often come out during high school or college age. At this age, they may not trust or ask for help from others, especially when their orientation is not accepted in society. Sometimes their own families are not even informed.
According to Rosario, Schrimshaw, Hunter, Braun (2006), "the development of a lesbian, gay, or bisexual (LGB) sexual identity is a complex and often difficult process. Unlike members of other minority groups (e.g., ethnic and racial minorities), most LGB individuals are not raised in a community of similar others from whom they learn about their identity and who reinforce and support that identity. Rather, LGB individuals are often raised in communities that are either ignorant of or openly hostile toward homosexuality."
Outing is the practice of publicly revealing the sexual orientation of a closeted person.  Notable politicians, celebrities, military service people, and clergy members have been outed, with motives ranging from malice to political or moral beliefs. Many commentators oppose the practice altogether, while some encourage outing public figures who use their positions of influence to harm other gay people. 
Lesbian narratives and sexual orientation awareness
Lesbians often experience their sexuality differently from gay men, and have different understandings about etiology from those derived from studies focused mostly on men. For information specific to female homosexuality, see Lesbian.
In a U.S.-based 1970s mail survey by Shere Hite, lesbians self-reported their reasons for being lesbian. This is the only major piece of research into female sexuality that has looked at how women understand being homosexual since Kinsey in 1953. The research yielded information about women's general understanding of lesbian relationships and their sexual orientation. Women gave various reasons for preferring sexual relations with women to sexual relations with men, including finding women more sensitive to other people's needs.
Since Hite carried out her study she has acknowledged that some women may have chosen the political identity of a lesbian. Julie Bindel, a UK journalist, reaffirmed that "political lesbianism continues to make intrinsic sense because it reinforces the idea that sexuality is a choice, and we are not destined to a particular fate because of our chromosomes." as recently as 2009. 
Early 20th-century writers on a homosexual orientation usually understood it to be intrinsically linked to the subject's own sex. For example, it was thought that a typical female-bodied person who is attracted to female-bodied persons would have masculine attributes, and vice versa. However, this understanding as sexual inversion was disputed at the time, and through the second half of the 20th century, gender identity came to be increasingly seen as a phenomenon distinct from sexual orientation.
Transgender and cisgender people may be attracted to men, women or both, although the prevalence of different sexual orientations is quite different in these two populations (see sexual orientation of transwomen). An individual homosexual, heterosexual or bisexual person may be masculine, feminine, or androgynous, and in addition, many members and supporters of lesbian and gay communities now see the "gender-conforming heterosexual" and the "gender-nonconforming homosexual" as negative stereotypes. However, studies by J. Michael Bailey and K.J. Zucker have found that a majority of gay men and lesbians report being gender-nonconforming during their childhood years.
Reliable data as to the size of the gay and lesbian population are of value in informing public policy.  For example, demographics would help in calculating the costs and benefits of domestic partnership benefits, of the impact of legalizing gay adoption, and of the impact of the U.S. military's Don't Ask Don't Tell policy.  Further, knowledge of the size of the "gay and lesbian population holds promise for helping social scientists understand a wide array of important questions—questions about the general nature of labor market choices, accumulation of human capital, specialization within households, discrimination, and decisions about geographic location." 
Measuring the prevalence of homosexuality presents difficulties. It is necessary to consider the measuring criteria that are used, the cutoff point and the time span taken to define a sexual orientation. Many people, despite having same-sex attractions, may be reluctant to identify themselves as gay or bisexual. The research must measure some characteristic that may or may not be defining of sexual orientation. The number of people with same-sex desires may be larger than the number of people who act on those desires, which in turn may be larger than the number of people who self-identify as gay, lesbian, or bisexual. 
In 1948 and 1953, Alfred Kinsey reported that nearly 46% of the male subjects had "reacted" sexually to persons of both sexes in the course of their adult lives, and 37% had had at least one homosexual experience.  Kinsey's methodology was criticized by John Tukey for using convenience samples and not random samples.  A later study tried to eliminate the sample bias, but still reached similar conclusions.  Simon LeVay cites these Kinsey results as an example of the caution needed to interpret demographic studies, as they may give quite differing numbers depending on what criteria are used to conduct them, in spite of using sound scientific methods.
According to major studies, 2% to 11% of people have had some form of same-sex sexual contact within their lifetime;         this percentage rises to 16–21% when either or both same-sex attraction and behavior are reported.  In a 2006 study, 20% of respondents anonymously reported some homosexual feelings, although only 2–3% identified themselves as homosexual. A 1992 study reported that 6.1% of males in Britain have had a homosexual experience, while in France the number was reported at 4.1%. 
In the United States, according to a report by The Williams Institute in April 2011, only 3.5% or approximately 9 million of the adult population are lesbian, gay, or bisexual.   According to the 2000 United States Census, there were about 601,209 same-sex unmarried partner households. 
A 2013 study by the CDC in which over 34,000 Americans were interviewed, puts the percentage of lesbians and gays at 1.6% and 0.7% as bisexual.
According to a 2008 poll, 13% of Britons have had some form of same-sex sexual contact while only 6% of Britons identify themselves as either homosexual or bisexual.  In contrast, a survey by the UK Office for National Statistics (ONS) in 2010 found that 95% of Britons identified as heterosexual, 1.5% of Britons identified themselves as homosexual or bisexual, and the last 3.5% gave more vague answers such as "don't know", "other", or did not respond to the question.  
An October 2012 Gallup poll provided unprecedented demographic information about those who identify as LGBT, arriving at the conclusion that 3.4%, with a margin of error of ±1%, of all U.S. adults identify as LGBT.  The study is the nation's largest in counting LGBT.  Gallup found that those 18-29 are about twice as likely as those 30-49 to identify as LGBT in the United States (6.4% to 3.2%, ±1%) and about three times as likely as those ages 65 or older. (6.4% to 1.9% ±1%) Among 18- to 29-year-olds, women were found to be almost twice as likely to identify as LGBT than men, 8.3% to 4.6%, ±1%; overall, there was no significant difference between the sexes, with 3.6% of women and 3.3% of men identifying as LGBT, ±1%. 
|Age/Gender||Yes||No|| Don't Know |
Psychology was one of the first disciplines to study a homosexual orientation as a discrete phenomenon. The first attempts to classify homosexuality as a disease were made by the fledgling European sexologist movement in the late 19th century. In 1886 noted sexologist Richard von Krafft-Ebing listed homosexuality along with 200 other case studies of deviant sexual practices in his definitive work, Psychopathia Sexualis . Krafft-Ebing proposed that homosexuality was caused by either "congenital [during birth] inversion" or an "acquired inversion". In the last two decades of the 19th century, a different view began to predominate in medical and psychiatric circles, judging such behavior as indicative of a type of person with a defined and relatively stable sexual orientation. In the late 19th century and early 20th century, pathological models of homosexuality were standard.
|“|| In 1952, when the American Psychiatric Association published its first Diagnostic and Statistical Manual of Mental Disorders, homosexuality was included as a disorder. Almost immediately, however, that classification began to be subjected to critical scrutiny in research funded by the National Institute of Mental Health. That study and subsequent research consistently failed to produce any empirical or scientific basis for regarding homosexuality as a disorder or abnormality, rather than a normal and healthy sexual orientation. As results from such research accumulated, professionals in medicine, mental health, and the behavioral and social sciences reached the conclusion that it was inaccurate to classify homosexuality as a mental disorder and that the DSM classification reflected untested assumptions based on once-prevalent social norms and clinical impressions from unrepresentative samples comprising patients seeking therapy and individuals whose conduct brought them into the criminal justice system. |
In recognition of the scientific evidence,  the American Psychiatric Association removed homosexuality from the DSM in 1973, stating that "homosexuality per se implies no impairment in judgment, stability, reliability, or general social or vocational capabilities." After thoroughly reviewing the scientific data, the American Psychological Association adopted the same position in 1975, and urged all mental health professionals "to take the lead in removing the stigma of mental illness that has long been associated with homosexual orientations." The National Association of Social Workers has adopted a similar policy.
Thus, mental health professionals and researchers have long recognized that being homosexual poses no inherent obstacle to leading a happy, healthy, and productive life, and that the vast majority of gay and lesbian people function well in the full array of social institutions and interpersonal relationships. 
 The longstanding consensus of research and clinical literature demonstrates that same-sex sexual and romantic attractions, feelings, and behaviors are normal and positive variations of human sexuality.  There is now a large body of research evidence that indicates that being gay, lesbian or bisexual is compatible with normal mental health and social adjustment.  The World Health Organization's ICD-9 (1977) listed homosexuality as a mental illness; it was removed from the ICD-10, endorsed by the Forty-third World Health Assembly on 17 May 1990.    Like the DSM-II, the ICD-10 added ego-dystonic sexual orientation to the list, which refers to people who want to change their gender identities or sexual orientation because of a psychological or behavioral disorder (). The Chinese Society of Psychiatry removed homosexuality from its Chinese Classification of Mental Disorders in 2001 after five years of study by the association. According to the Royal College of Psychiatrists "This unfortunate history demonstrates how marginalisation of a group of people who have a particular personality feature (in this case homosexuality) can lead to harmful medical practice and a basis for discrimination in society.  There is now a large body of research evidence that indicates that being gay, lesbian or bisexual is compatible with normal mental health and social adjustment. However, the experiences of discrimination in society and possible rejection by friends, families and others, such as employers, means that some LGB people experience a greater than expected prevalence of mental health difficulties and substance misuse problems. Although there have been claims by conservative political groups in the USA that this higher prevalence of mental health difficulties is confirmation that homosexuality is itself a mental disorder, there is no evidence whatever to substantiate such a claim." 
Most lesbian, gay, and bisexual people who seek psychotherapy do so for the same reasons as heterosexual people (stress, relationship difficulties, difficulty adjusting to social or work situations, etc.); their sexual orientation may be of primary, incidental, or no importance to their issues and treatment. Whatever the issue, there is a high risk for anti-gay bias in psychotherapy with lesbian, gay, and bisexual clients. Psychological research in this area has been relevant to counteracting prejudicial (" homophobic ") attitudes and actions, and to the LGBT rights movement generally.
The appropriate application of affirmative psychotherapy is based on the following scientific facts: 
- Same-sex sexual attractions, behavior, and orientations per se are normal and positive variants of human sexuality; in other words, they are not indicators of mental or developmental disorders.
- Homosexuality and bisexuality are stigmatized, and this stigma can have a variety of negative consequences (e.g., Minority Stress) throughout the life span (D'Augelli & Patterson, 1995; DiPlacido, 1998; Herek & Garnets, 2007; Meyer, 1995, 2003).
- Same-sex sexual attractions and behavior can occur in the context of a variety of sexual orientations and sexual orientation identities (Diamond, 2006; Hoburg et al., 2004; Rust, 1996; Savin-Williams, 2005).
- Gay men, lesbians, and bisexual individuals can live satisfying lives as well as form stable, committed relationships and families that are equivalent to heterosexual relationships in essential respects (APA, 2005c; Kurdek, 2001, 2003, 2004; Peplau & Fingerhut, 2007).
- There are no empirical studies or peer-reviewed research that support theories attributing same-sex sexual orientation to family dysfunction or trauma (Bell et al., 1981; Bene, 1965; Freund & Blanchard, 1983; Freund & Pinkava, 1961; Hooker, 1969; McCord et al., 1962; D. K. Peters & Cantrell, 1991; Siegelman, 1974, 1981; Townes et al., 1976).
Biological vs environmental determinants
Although scientists favor biological models for the cause of sexual orientation,  they do not believe that it is the result of any one factor. They generally believe that it is determined by biological and environmental factors ; they state that most people's sexual orientation is determined at an early age, and sexual orientation development involves a complex interplay between nature and nurture.    The biological factors are genetic and hormonal, both of which affect the fetal development of the brain, while environmental factors may be sociological, psychological, or involve the early uterine environment.    Scientists generally do not believe that sexual orientation is a matter of choice.  
|“||There is no scientific evidence that abnormal parenting, sexual abuse, or other adverse life events influence sexual orientation. Current knowledge suggests that sexual orientation is usually established during early childhood. ||”|
|“||Currently, there is no scientific consensus about the specific factors that cause an individual to become heterosexual, homosexual, or bisexual—including possible biological, psychological, or social effects of the parents' sexual orientation. However, the available evidence indicates that the vast majority of lesbian and gay adults were raised by heterosexual parents and the vast majority of children raised by lesbian and gay parents eventually grow up to be heterosexual. ||”|
Despite numerous attempts, no "gay gene" has been identified. However, there is substantial evidence for a genetic basis of homosexuality especially in males based on twin studies, with some association with regions of Chromosome 8 and with the Xq28 gene on the X chromosome.  More recently, epigenetics has been implicated in sexual orientiation. In five regions of the genome the methylation pattern appears very closely linked to sexual orientation. The methylation pattern predicted the sexual orientation of a control group with almost 70% accuracy. 
Since homosexuality tends to lower reproductive success, and since there is considerable evidence that human sexual orientation is genetically influenced, it is unclear how it is maintained in the population at a relatively high frequency. There are many possible explanations, such as genes predisposing to homosexuality also conferring advantage in heterosexuals, a kin selection effect, social prestige, and more.  A 2009 study also suggested a significant increase in fecundity in the females related to the homosexual people from the maternal line (but not in those related from the paternal one). 
Sexual orientation change efforts
There are no studies of adequate scientific rigor that conclude that sexual orientation change efforts work to change a person's sexual orientation. Those efforts have been controversial due to tensions between the values held by some faith-based organizations, on the one hand, and those held by LGBT rights organizations and professional and scientific organizations and other faith-based organizations, on the other.  The longstanding consensus of the behavioral and social sciences and the health and mental health professions is that homosexuality per se is a normal and positive variation of human sexual orientation, and therefore not a mental disorder.  The American Psychological Association says that "most people experience little or no sense of choice about their sexual orientation".  Some individuals and groups have promoted the idea of homosexuality as symptomatic of developmental defects or spiritual and moral failings and have argued that sexual orientation change efforts, including psychotherapy and religious efforts, could alter homosexual feelings and behaviors. Many of these individuals and groups appeared to be embedded within the larger context of conservative religious political movements that have supported the stigmatization of homosexuality on political or religious grounds. 
No major mental health professional organization has sanctioned efforts to change sexual orientation and virtually all of them have adopted policy statements cautioning the profession and the public about treatments that purport to change sexual orientation. These include the American Psychiatric Association, American Psychological Association, American Counseling Association, National Association of Social Workers in the USA,  the Royal College of Psychiatrists,  and the Australian Psychological Society.  The American Psychological Association and the Royal College of Psychiatrists expressed concerns that the positions espoused by NARTH are not supported by the science and create an environment in which prejudice and discrimination can flourish.  
The American Psychological Association states that "sexual orientation is not a choice that can be changed at will, and that sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors...is shaped at an early age...[and evidence suggests] biological, including genetic or inborn hormonal factors, play a significant role in a person's sexuality."  They say that "sexual orientation identity—not sexual orientation—appears to change via psychotherapy, support groups, and life events."  The American Psychiatric Association says "individuals maybe become aware at different points in their lives that they are heterosexual, gay, lesbian, or bisexual" and "opposes any psychiatric treatment, such as 'reparative' or 'conversion' therapy, which is based upon the assumption that homosexuality per se is a mental disorder, or based upon a prior assumption that the patient should change his/her homosexual orientation". They do, however, encourage gay affirmative psychotherapy. 
The American Psychological Association "encourages mental health professionals to avoid misrepresenting the efficacy of sexual orientation change efforts by promoting or promising change in sexual orientation when providing assistance to individuals distressed by their own or others' sexual orientation and concludes that the benefits reported by participants in sexual orientation change efforts can be gained through approaches that do not attempt to change sexual orientation". 
Scientific research has been generally consistent in showing that lesbian and gay parents are as fit and capable as heterosexual parents, and their children are as psychologically healthy and well-adjusted as children reared by heterosexual parents.    According to scientific literature reviews, there is no evidence to the contrary.     
A review study suggested that the children with lesbian or gay parents appear less traditionally gender-typed and are more likely to be open to homoerotic relationships, partly due to genetic (80% of the children being raised by same-sex couples in the US are not adopted and most are the result of heterosexual marriages)  ) and family socialization processes (children grow up in relatively more tolerant school, neighborhood, and social contexts, which are less heterosexist), even though majority of children raised by same-sex couples identify as heterosexual.  A 2005 review by Charlotte J. Patterson for the American Psychological Association found that the available data did not suggest higher rates of homosexuality among the children of lesbian or gay parents.  One study suggested that children of gay and lesbian parents were more likely to adopt non-heterosexual identities, especially daughters of lesbian parents (inter-generational transfer was not significant in some analyses for sons). 
The terms "Men who have sex with men" (MSM) and " women who have sex with women " (WSW) refer to people who engage in sexual activity with others of the same sex regardless of how they identify themselves—as many choose not to accept social identities as lesbian, gay and bisexual.     These terms are often used in medical literature and social research to describe such groups for study, without needing to consider the issues of sexual self-identity. The terms are seen as problematic, however, because they "obscure social dimensions of sexuality; undermine the self-labeling of lesbian, gay, and bisexual people; and do not sufficiently describe variations in sexual behavior".  MSM and WSW are sexually active with each other for a variety of reasons with the main ones arguably sexual pleasure, intimacy and bonding. In contrast to its benefits, sexual behavior can be a disease vector. Safe sex is a relevant harm reduction philosophy.  The United States currently prohibits men who have sex with men from donating blood "because they are, as a group, at increased risk for HIV, hepatitis B and certain other infections that can be transmitted by transfusion." Many European countries have the same prohibition. 
- Avoid contact with a partner's menstrual blood and with any visible genital lesions.
- Cover sex toys that penetrate more than one person's vagina or anus with a new condom for each person; consider using different toys for each person.
- Use a barrier (e.g., latex sheet, dental dam, cut-open condom, plastic wrap) during oral sex.
- Use latex or vinyl gloves and lubricant for any manual sex that might cause bleeding.
These safer sex recommendations are agreed upon by public health officials for men who have sex with men to avoid sexually transmitted infections:
- Avoid contact with a partner's bodily fluids and with any visible genital lesions.
- Use condoms for anal and oral sex.
- Use a barrier (e.g., latex sheet, dental dam, cut-open condom) during anal–oral sex.
- Cover sex toys that penetrate more than one person with a new condom for each person; consider using different toys for each person and use latex or vinyl gloves and lubricant for any sex that might cause bleeding.
When it was first described in medical literature, homosexuality was often approached from a view that sought to find an inherent psychopathology as its root cause. Much literature on mental health and homosexual patients centered on their depression, substance abuse, and suicide. Although these issues exist among people who are non-heterosexual, discussion about their causes shifted after homosexuality was removed from the Diagnostic and Statistical Manual (DSM) in 1973. Instead, social ostracism, legal discrimination, internalization of negative stereotypes, and limited support structures indicate factors homosexual people face in Western societies that often adversely affect their mental health. Stigma, prejudice, and discrimination stemming from negative societal attitudes toward homosexuality lead to a higher prevalence of mental health disorders among lesbians, gay men, and bisexuals compared to their heterosexual peers.  Evidence indicates that the liberalization of these attitudes over the past few decades is associated with a decrease in such mental health risks among younger LGBT people. 
Gay and lesbian youth
Gay and lesbian youth bear an increased risk of suicide, substance abuse, school problems, and isolation because of a "hostile and condemning environment, verbal and physical abuse, rejection and isolation from family and peers". Further, LGBT youths are more likely to report psychological and physical abuse by parents or caretakers, and more sexual abuse. Suggested reasons for this disparity are that (1) LGBT youths may be specifically targeted on the basis of their perceived sexual orientation or gender non-conforming appearance, and (2) that "risk factors associated with sexual minority status, including discrimination, invisibility, and rejection by family members...may lead to an increase in behaviors that are associated with risk for victimization, such as substance abuse, sex with multiple partners, or running away from home as a teenager."  A 2008 study showed a correlation between the degree of rejecting behavior by parents of LGB adolescents and negative health problems in the teenagers studied:
Higher rates of family rejection were significantly associated with poorer health outcomes. On the basis of odds ratios, lesbian, gay, and bisexual young adults who reported higher levels of family rejection during adolescence were 8.4 times more likely to report having attempted suicide, 5.9 times more likely to report high levels of depression, 3.4 times more likely to use illegal drugs, and 3.4 times more likely to report having engaged in unprotected sexual intercourse compared with peers from families that reported no or low levels of family rejection. 
Crisis centers in larger cities and information sites on the Internet have arisen to help youth and adults.  The Trevor Project, a suicide prevention helpline for gay youth, was established following the 1998 airing on HBO of the Academy Award winning short film Trevor . 
Law and politics
Most nations do not prohibit consensual sex between unrelated persons above the local age of consent. Some jurisdictions further recognize identical rights, protections, and privileges for the family structures of same-sex couples, including marriage. Some nations mandate that all individuals restrict themselves to heterosexual relationships; that is, in some jurisdictions homosexual activity is illegal. Offenders can face the death penalty in some fundamentalist Muslim areas such as Iran and parts of Nigeria. There are, however, often significant differences between official policy and real-world enforcement. See Violence against LGBT people .
Although homosexual acts were decriminalized in some parts of the Western world, such as Poland in 1932, Denmark in 1933, Sweden in 1944, and the United Kingdom in 1967, it was not until the mid-1970s that the gay community first began to achieve limited civil rights in some developed countries. A turning point was reached in 1973 when the American Psychiatric Association removed homosexuality from the Diagnostic and Statistical Manual of Mental Disorders , thus negating its previous definition of homosexuality as a clinical mental disorder. In 1977, Quebec became the first state-level jurisdiction in the world to prohibit discrimination on the grounds of sexual orientation. During the 1980s and 1990s, most developed countries enacted laws decriminalizing homosexual behavior and prohibiting discrimination against lesbian and gay people in employment, housing, and services. On the other hand, many countries today in the Middle East and Africa, as well as several countries in Asia, the Caribbean and the South Pacific, outlaw homosexuality. On 11 December 2013, homosexuality was criminalized in India by a Supreme Court ruling. The Section 377 of the colonial-era Indian Penal Code which criminalizes homosexuality remains in effect in many former colonies.  In six countries, homosexual behavior is punishable by life imprisonment ; in ten others, it carries the death penalty. 
Laws against sexual orientation discrimination
- Employment discrimination refers to discriminatory employment practices such as bias in hiring, promotion, job assignment, termination, and compensation, and various types of harassment. In the United States there is "very little statutory, common law, and case law establishing employment discrimination based upon sexual orientation as a legal wrong." Some exceptions and alternative legal strategies are available. President Bill Clinton's Executive Order 13087 (1998) prohibits discrimination based on sexual orientation in the competitive service of the federal civilian workforce,  and federal non-civil service employees may have recourse under the Due Process Clause of the U.S. Constitution. Private sector workers may have a Title VII action under a quid pro quo sexual harassment theory, a "hostile work environment" theory,  a sexual stereotyping theory,  or others.
- Housing discrimination refers to discrimination against potential or current tenants by landlords. In the United States, there is no federal law against such discrimination on the basis of sexual orientation or gender identity, but at least thirteen states and many major cities have enacted laws prohibiting it. 
- Hate crimes (also known as bias crimes ) are crimes motivated by bias against an identifiable social group, usually groups defined by race (human classification), religion, sexual orientation, disability, ethnicity, nationality, age, gender, gender identity, or political affiliation. In the United States, 45 states and the District of Columbia have statutes criminalizing various types of bias-motivated violence or intimidation (the exceptions are AZ, GA, IN, SC, and WY). Each of these statutes covers bias on the basis of race, religion, and ethnicity; 32 of them cover sexual orientation, 28 cover gender, and 11 cover transgender/gender-identity.  In October 2009, the Matthew Shepard and James Byrd, Jr. Hate Crimes Prevention Act, which "...gives the Justice Department the power to investigate and prosecute bias-motivated violence where the perpetrator has selected the victim because of the person's actual or perceived race, color, religion, national origin, gender, sexual orientation, gender identity or disability", was signed into law and makes hate crime based on sexual orientation, amongst other offenses, a federal crime in the United States. 
In the European Union, discrimination of any type based on sexual orientation or gender identity is illegal under the Charter of Fundamental Rights of the European Union.
Since the 1960s, many LGBT people in the West, particularly those in major metropolitan areas, have developed a so-called gay culture. To many, gay culture is exemplified by the gay pride movement, with annual parades and displays of rainbow flags. Yet not all LGBT people choose to participate in "queer culture", and many gay men and women specifically decline to do so. To some it seems to be a frivolous display, perpetuating gay stereotypes. To some others, the gay culture represents heterophobia and is scorned as widening the gulf between gay and non-gay people.
With the outbreak of AIDS in the early 1980s, many LGBT groups and individuals organized campaigns to promote efforts in AIDS education, prevention, research, patient support, and community outreach, as well as to demand government support for these programs.
The bewildering death toll wrought by the AIDS epidemic at first seemed to slow the progress of the gay rights movement, but in time it galvanized some parts of the LGBT community into community service and political action, and challenged the heterosexual community to respond compassionately. Major American motion pictures from this period that dramatized the response of individuals and communities to the AIDS crisis include An Early Frost (1985), Longtime Companion (1990), And the Band Played On (1993), Philadelphia (1993), and Common Threads: Stories from the Quilt (1989).
Publicly gay politicians have attained numerous government posts, even in countries that had sodomy laws in their recent past. Examples include Guido Westerwelle, Germany's Vice-Chancellor ; Peter Mandelson, a British Labour Party cabinet minister and Per-Kristian Foss, formerly Norwegian Minister of Finance.
LGBT movements are opposed by a variety of individuals and organizations. Some social conservatives believe that all sexual relationships with people other than an opposite-sex spouse undermine the traditional family  and that children should be reared in homes with both a father and a mother.   Some opponents of gay rights say that such rights may conflict with individuals' freedom of speech,      religious freedoms in the workplace,   the ability to run churches,  charitable organizations   and other religious organizations  in accordance with one's religious views, and that the acceptance of homosexual relationships by religious organizations might be forced through threatening to remove the tax-exempt status of churches whose views do not align with those of the government.     Some critics charge that political correctness has led to the association of sex between males and HIV being downplayed.  
Policies and attitudes toward gay and lesbian military personnel vary widely around the world. Some countries allow gay men, lesbians, and bisexual people to serve openly and have granted them the same rights and privileges as their heterosexual counterparts. Many countries neither ban nor support LGB service members. A few countries continue to ban homosexual personnel outright.
Most Western military forces have removed policies excluding sexual minority members. Of the 26 countries that participate militarily in NATO, more than 20 permit openly gay, lesbian and bisexual people to serve. Of the permanent members of the United Nations Security Council, three ( United Kingdom, France and United States) do so. The other two generally do not: China bans gay and lesbian people outright, Russia excludes all gay and lesbian people during peacetime but allows some gay men to serve in wartime (see below). Israel is the only country in the Middle East region that allows openly LGB people to serve in the military.
While the question of homosexuality in the military has been highly politicized in the United States, it is not necessarily so in many countries. Generally speaking, sexuality in these cultures is considered a more personal aspect of one's identity than it is in the United States.
According to American Psychological Association empirical evidence fails to show that sexual orientation is germane to any aspect of military effectiveness including unit cohesion, morale, recruitment and retention.  Sexual orientation is irrelevant to task cohesion, the only type of cohesion that critically predicts the team's military readiness and success. 
Society and sociology
Societal acceptance of non-heterosexual orientations such as homosexuality is lowest in Asian and African countries, and is highest in Europe, Australia, and the Americas. Western society has become increasingly accepting of homosexuality over the past few decades. In 2017, Professor Amy Adamczyk contended that these cross-national differences in acceptance can be largely explained by three factors: the relative strength of democratic institutions, the level of economic development, and the religious context of the places where people live. 
In 2006, the American Psychological Association, American Psychiatric Association and National Association of Social Workers stated in an amicus brief presented to the Supreme Court of the State of California: "Gay men and lesbians form stable, committed relationships that are equivalent to heterosexual relationships in essential respects. The institution of marriage offers social, psychological, and health benefits that are denied to same-sex couples. By denying same-sex couples the right to marry, the state reinforces and perpetuates the stigma historically associated with homosexuality. Homosexuality remains stigmatized, and this stigma has negative consequences. California's prohibition on marriage for same-sex couples reflects and reinforces this stigma". They concluded: "There is no scientific basis for distinguishing between same-sex couples and heterosexual couples with respect to the legal rights, obligations, benefits, and burdens conferred by civil marriage." 
Though the relationship between homosexuality and religion can vary greatly across time and place, within and between different religions and sects, and regarding different forms of homosexuality and bisexuality, current authoritative bodies and doctrines of the world's largest religions generally view homosexuality negatively. This can range from quietly discouraging homosexual activity, to explicitly forbidding same-sex sexual practices among adherents and actively opposing social acceptance of homosexuality. Some teach that homosexual orientation itself is sinful,  others state that only the sexual act is a sin,  others are completely accepting of gays and lesbians,  while some encourage homosexuality.  Some claim that homosexuality can be overcome through religious faith and practice. On the other hand, voices exist within many of these religions that view homosexuality more positively, and liberal religious denominations may bless same-sex marriages. Some view same-sex love and sexuality as sacred, and a mythology of same-sex love can be found around the world. 
Gay bullying can be the verbal or physical abuse against a person who is perceived by the aggressor to be lesbian, gay, bisexual or transgender, including persons who are actually heterosexual or of non-specific or unknown sexual orientation. In the US, teenage students heard anti-gay slurs such as "homo", "faggot" and "sissy" about 26 times a day on average, or once every 14 minutes, according to a 1998 study by Mental Health America (formerly National Mental Health Association). 
Heterosexism and homophobia
In many cultures, homosexual people are frequently subject to prejudice and discrimination. A 2011 Dutch study concluded that 49% of Holland's youth and 58% of youth foreign to the country reject homosexuality.  Similar to other minority groups they can also be subject to stereotyping. These attitudes tend to be due to forms of homophobia and heterosexism (negative attitudes, bias, and discrimination in favor of opposite-sex sexuality and relationships). Heterosexism can include the presumption that everyone is heterosexual or that opposite-sex attractions and relationships are the norm and therefore superior. Homophobia is a fear of, aversion to, or discrimination against homosexual people. It manifests in different forms, and a number of different types have been postulated, among which are internalized homophobia, social homophobia, emotional homophobia, rationalized homophobia, and others.  Similar is lesbophobia (specifically targeting lesbians) and biphobia (against bisexual people). When such attitudes manifest as crimes they are often called hate crimes and gay bashing.
Negative stereotypes characterize LGB people as less romantically stable, more promiscuous and more likely to abuse children, but there is no scientific basis to such assertions. Gay men and lesbians form stable, committed relationships that are equivalent to heterosexual relationships in essential respects.  Sexual orientation does not affect the likelihood that people will abuse children.    Claims that there is scientific evidence to support an association between being gay and being a pedophile are based on misuses of those terms and misrepresentation of the actual evidence. 
Violence against gays and lesbians
In the United States, the FBI reported that 20.4% of hate crimes reported to law enforcement in 2011 were based on sexual orientation bias. 56.7% of these crimes were based on bias against homosexual men. 11.1% were based on bias against homosexual women. 29.6% were based on anti-homosexual bias without regard to gender. The 1998 murder of Matthew Shepard, a gay student, is a notorious such incident in the U.S. LGBT people, especially lesbians, may become the victims of " corrective rape ", a violent crime with the supposed aim of making them heterosexual. In certain parts of the world, LGBT people are also at risk of " honor killings " perpetrated by their families or relatives.   
Homosexual behavior in other animals
Homosexual and bisexual behaviors occur in a number of other animal species. Such behaviors include sexual activity, courtship, affection, pair bonding, and parenting, and are widespread; a 1999 review by researcher Bruce Bagemihl shows that homosexual behavior has been documented in about 500 species, ranging from primates to gut worms.  Animal sexual behavior takes many different forms, even within the same species. The motivations for and implications of these behaviors have yet to be fully understood, since most species have yet to be fully studied.  According to Bagemihl, "the animal kingdom [does] it with much greater sexual diversity—including homosexual, bisexual and nonreproductive sex—than the scientific community and society at large have previously been willing to accept". 
A review paper by N. W. Bailey and Marlene Zuk looking into studies of same-sex sexual behaviour in animals challenges the view that such behaviour lowers reproductive success, citing several hypotheses about how same-sex sexual behavior might be adaptive; these hypotheses vary greatly among different species. Bailey and Zuk also suggest future research needs to look into evolutionary consequences of same-sex sexual behaviour, rather than only looking into origins of such behaviour. 
- LGBT rights by country or territory
- LGBT rights at the United Nations
- Anti-LGBT rhetoric
- Biology and sexual orientation
- Fraternal birth order and male sexual orientation
- Gay sexual practices
- Gender dysphoria
- Hate speech
- Human male sexuality
- List of nonfiction books about homosexuality
- List of gay, lesbian or bisexual people
- Non-westernized concepts of male sexuality
- Religion and sexuality
- Riddle homophobia scale
- Sexuality and gender identity-based cultures
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