Heterosexuality is romantic attraction, sexual attraction or sexual behavior between persons of the opposite sex or gender . As a sexual orientation , heterosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" to persons of the opposite 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 homosexuality , heterosexuality is one of the three main categories of sexual orientation within the heterosexual–homosexual continuum .  Someone who is heterosexual is commonly referred to as straight.
Hetero- comes from the Greek word έτερος [héteros], meaning "other party" or "another", used in science as a prefix meaning "different";  and the Latin word for sex (that is, characteristic sex or sexual differentiation ). The term " heterosexual " was first published in 1892 in C.G. Chaddock's translation of Krafft-Ebing's " Psychopathia Sexualis ". The noun came into use from the early 1920s, but did not enter common use until the 1960s. The colloquial shortening "hetero" is attested from 1933. The abstract noun "heterosexuality" is first recorded in 1900. The word "heterosexual" was first listed in Merriam-Webster's New International Dictionary as a medical term for "morbid sexual passion for one of the opposite sex"; however, in 1934 in their Second Edition Unabridged it is defined as a "manifestation of sexual passion for one of the opposite sex; normal sexuality". The adjective heterosexual is used for intimate relationships or sexual relations between male and female.
The current use of the term heterosexual has its roots in the broader 19th century tradition of personality taxonomy. It continues to influence the development of the modern concept of sexual orientation, and can be used to describe individuals' sexual orientation, sexual history, or self-identification. Some reject the term heterosexual , as they feel that the word only refers to one's sexual behavior and does not refer to non-sexual romantic feelings. The term heterosexual is suggested to have come into use as a neologism after, and opposite to, the word homosexual by Karl Maria Kertbeny in 1868. In LGBT slang, the term breeder has been used as a denigrating phrase to deride heterosexuals. Hyponyms of heterosexual include heteroflexible .
The word can be informally  shortened to "hetero".  The term straight originated as a mid-20th century gay slang term for heterosexuals, ultimately coming from the phrase "to go straight" (as in "straight and narrow"), or stop engaging in homosexual sex. One of the first uses of the word in this way was in 1941 by author G. W. Henry. Henry's book concerned conversations with homosexual males and used this term in connection with people who are identified as ex-gays . It is now simply a colloquial term for "heterosexual", having changed in primary meaning over time. Some object to usage of the term straight because it implies that non-heteros are crooked.
Heterosexual symbolism dates back to the earliest artifacts of humanity, with gender symbols , ritual fertility carvings, and primitive art. This was later expressed in the symbolism of fertility rites and polytheistic worship , which often included images of human reproductive organs , such as lingam in Hinduism . Modern symbols of heterosexuality in societies derived from European traditions still reference symbols used in these ancient beliefs. One such image is a combination of the symbol for Mars , the Roman god of war, as the definitive male symbol of masculinity, and Venus , the Roman goddess of love and beauty, as the definitive female symbol of femininity. The unicode character for this combined symbol is ⚤ (U+26A4).
The Judeo-Christian tradition has several scriptures related to heterosexuality. The Genesis states that God created man because "it is not good that the man should be alone; I will make him an help meet for him." ( ) Genesis then contains a commandment stating "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh" ( ) In 1 Corinthians , Christians are advised:
Now for the matters you wrote about: It is good for a man not to marry. But since there is so much immorality, each man should have his own wife, and each woman her own husband. The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. The wife's body does not belong to her alone but also to her husband. In the same way, the husband's body does not belong to him alone but also to his wife. Do not deprive each other except by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control. I say this as a concession, not as a command. (NIV) 
For the most part, religious traditions in the world reserve marriage to heterosexual unions, but there are exceptions including certain Buddhist and Hindu traditions, Unitarian Universalist , Metropolitan Community Church and some Anglican dioceses and some Quaker , United Church of Canada and Reform and Conservative Jewish congregations.  
Almost all religions believe that lawful sex between a man and a woman is allowed, but there are a few that believe that it is a sin, such as The Shakers , The Harmony Society , and The Ephrata Cloister . These religions tend to view all sexual relations as sinful , and promote celibacy . Other religions view heterosexual relationships as being inferior to celibacy. Some religions require celibacy for certain roles, such as Catholic priests ; however, the Catholic Church also views heterosexual marriage as sacred and necessary. 
The demographics of sexual orientation are difficult to establish due to a lack of reliable data. However, the history of human sexuality shows that attitudes and behavior have varied across societies. According to major studies, 89% to 98% of people have had only heterosexual contact within their lifetime;   but this percentage falls to 79–84% when either or both same-sex attraction and behavior are reported.  In a 2006 study, 80% of respondents anonymously reported heterosexual feelings, although 97–98% identified themselves as heterosexual. A 1992 study reported that 93.9% of males in Britain have always had heterosexual experience, while in France the number was reported at 95.9%.
According to a 2008 poll, 85% of Britons have only opposite-sex sexual contact while only 94% of Britons identify themselves as heterosexual.  Similarly, 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 heterosexual, arriving at the conclusion that 96.6%, with a margin of error of ±1%, of all U.S. adults identify as heterosexual. 
In a 2015 Yougov survey of 1,632 adults of the United Kingdom , 88.7% identified as heterosexual, 5.5% as homosexual and 2.1% as bisexual.  Asked to place themselves on the Kinsey scale , 72% of all adults, and 46% of adults aged 18–24 years, picked a score of zero, meaning that they identify as totally heterosexual. 4% of the total sample, and 6% of young adults, picked a score of six, meaning a totally homosexual identity. 
In another Yougov survey of 1,000 adults of the United States , 89% of the sample identified as heterosexual, 4% as homosexual (among 2% as homosexual male and 2% as homosexual female) and 4% as bisexual (of either sex). 
Biological and environmental
The relationship between biology and sexual orientation is a subject of research. No simple and singular determinant for sexual orientation has been conclusively demonstrated; various studies point to different, even conflicting positions, but scientists hypothesize that a combination of genetic, hormonal, and social factors determine sexual orientation.    Biological theories for explaining the causes of sexual orientation are more popular,  and biological factors may involve a complex interplay of genetic factors and the early uterine environment, or biological and social factors.    These factors, which may be related to the development of heterosexual or other orientation, include genes , prenatal hormones , and brain structure and their interaction with the environment.
Prenatal hormonal theory
The neurobiology of the masculinization of the brain is fairly well understood. Estradiol and testosterone , which is catalyzed by the enzyme 5α-reductase into dihydrotestosterone , act upon androgen receptors in the brain to masculinize it. If there are few androgen receptors (people with androgen insensitivity syndrome ) or too much androgen (females with congenital adrenal hyperplasia ), there can be physical and psychological effects. It has been suggested that both male and female heterosexuality are results of variation in this process. In these studies heterosexuality in females is linked to a lower amount of masculinization than is found in lesbian females, though when dealing with male heterosexuality there are results supporting both higher and lower degrees of masculinization than homosexual males.
Heterosexual behaviors in animals
Most sexual reproduction in the animal world is facilitated through opposite-sex sexual activity, although there are also animals that reproduce asexually , including protozoa and lower invertebrates . 
Reproductive sex does not necessarily require a heterosexual orientation, since orientation refers to a long-term enduring pattern of sexual and emotional attraction leading often to long-term social bonding, while reproductive sex requires only the basic act of intercourse only to fertile the ovum by sperm , often done one time only.   
At the beginning of the 20th century, early theoretical discussions in the field of psychoanalysis posited original bisexuality in human psychological development. Quantitative studies by Alfred Kinsey in the 1940s and Dr. Fritz Klein 's sexual orientation grid in the 1980s find distributions similar to those postulated by their predecessors.
According to Sexual Behavior in the Human Male by Alfred Kinsey and several other modern studies, the majority of humans have had both heterosexual and homosexual experiences or sensations and are bisexual. Kinsey himself, along with current sex therapists, focused on the historicity and fluidity of sexual orientation. Kinsey's studies consistently found sexual orientation to be something that evolves in many directions over a person's lifetime; rarely, but not necessarily, including forming attractions to a new sex. Rarely do individuals radically reorient their sexualities rapidly—and still less do they do so volitionally—but often sexualities expand, shift, and absorb new elements over decades. For example, socially normative "age-appropriate" sexuality requires a shifting object of attraction (especially in the passage through adolescence). Contemporary queer theory , incorporating many ideas from social constructionism , tends to look at sexuality as something that has meaning only within a given historical framework. Sexuality, then, is seen as a participation in a larger social discourse and, though in some sense fluid, not as something strictly determinable by the individual.
Sexologists have attributed discrepancies in some findings to negative societal attitudes towards a particular sexual orientation. For example, people may state different sexual orientations depending on whether their immediate social environment is public or private. Reluctance to disclose one's actual sexual orientation is often referred to as "being in the closet." Individuals capable of enjoyable sexual relations with both sexes or one sex may feel inclined to restrict themselves to heterosexual or homosexual relations in societies that stigmatize same-sex or opposite-sex relations.
Nature and nurture
The considerable " nature and nurture " debate exists over whether predominantly biological or psychological factors produce sexual orientation in humans, or whether both significantly factor into sexual orientation. Candidate factors include genes , the exposure of fetuses to certain hormones (or lack thereof) and environmental factors .
Critique of studies
The studies performed in order to find the origin of sexual orientation have been criticized for being too limited in scope, mostly for focusing only on heterosexuality and homosexuality as two diametrically opposite poles with no orientation in between. It is also asserted that scientific studies focus too much on the search for a biological explanation for sexual orientation, and not enough on the combined effects of both biology and psychology. 
In a brief by the Council for Responsible Genetics ,  it was stated that sexual orientation is not fixed either way, and on the discourse over sexual orientation: "Noticeably missing from this debate is the notion, championed by Kinsey, that human sexual expression is as variable among people as many other complex traits. Yet just like intelligence, sexuality is a complex human feature that modern science is attempting to explain with genetics. Research on brain size, hormone levels, finger length, and other biological traits have yet to yield evidence for this, however. It is important to note that traits such as these result from a combination of gene expression and developmental and other environmental factors. Well-known biologist and social theorist, Anne Fausto-Sterling advocates in her book Sexing the Body, for what scientists term a “systems approach” to be applied to our understanding of sexual preference. Rather than determining that this results from purely biological processes, a trait evolves from developmental processes that include both biological and social elements."  According to the American Psychological Association (APA), there are numerous theories about the origins of a person's sexual orientation, but some believe that "sexual orientation is most likely the result of a complex interaction of environmental, cognitive and biological factors," and that genetic factors play a "significant role" in determining a person's sexuality. 
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." 
In a 2004 study, the female subjects (both gay and straight women) became strongly sexually aroused when they viewed heterosexual as well as lesbian erotic films. Among the male subjects, however, the straight men were more turned on by erotic films with women, the gay ones more 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. 
Heteroflexibility is a form of a sexual orientation or situational sexual behavior characterized by minimal homosexual activity in an otherwise primarily heterosexual orientation that is considered to distinguish it from bisexuality. It has been characterized as "mostly straight". 
Sexual orientation change efforts
Sexual orientation change efforts are methods that aim to change sexual orientation, used to try to convert homosexual and bisexual people to heterosexuality. Scientists and mental health professionals generally do not believe that sexual orientation is a choice.   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. 
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 states that "sexual orientation is not a choice that can be changed at will",  and "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". While opposing conversion therapy, they encourage gay affirmative psychotherapy  and "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".  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.  
Social and historical
Since the 1960s and 1970s, a large body of research has provided evidence and analysis of the extent to which heterosexuality and homosexuality are socially organized and historically changing.  This work challenges the assumption that heterosexuality, homosexuality, and sexualities of all varieties, can be understood as primarily biological and psychological phenomena.
A heterosexual couple, a man and woman in an intimate relationship , form the core of a nuclear family .  Many societies throughout history have insisted that a marriage take place before the couple settle down, but enforcement of this rule or compliance with it has varied considerably. In some jurisdictions, when an unmarried man and woman live together long enough, they are deemed to have established a common-law marriage .
Social history and terminology
There was no real need to coin a term such as "heterosexual" until there was something else to contrast and compare it with. In “The Invention of Heterosexuality,” Jonathon Ned Katz dates the definition of heterosexuality, as it is used today, to the late 19th century.  In the Victorian era , sex was seen as a means to achieve reproduction, relations between the sexes were not believed to be overtly sexual. The body was thought of as a tool for procreation, “human energy, though of as a closed and severely limited system, was to be used in producing children and in work, not wasted in libidinous pleasures.”  Modern ideas of sexuality and eroticism began to develop in America and Germany in the later 19th century. The changing economy and the “transformation of the family from producer to consumer”  resulted in shifting values. The Victorian work ethic had changed, pleasure became more highly valued and this allowed ideas of human sexuality to change. Consumer culture had created a market for the erotic, pleasure became commoditized . At the same time medical doctors began to acquire more power and influence. They developed the medical model of Normal Love in which healthy men and women enjoyed sex as part of a “new ideal of male-female relationships that included.. an essential, necessary, normal eroticism.”  This ‘Normal Sexual’ ideal also had a counterpart, the Victorian Sex Pervert, anyone who failed to meet the norm . The basic oppositeness of the sexes was the basis for normal, healthy sexual attraction. “The attention paid the sexual abnormal created a need to name the sexual normal, the better to distinguish the average him and her from the deviant it.”  The creation term ‘heterosexual’ consolidated the social existence of the pre-existing heterosexual experience and created a sense of ensured and validated normalcy within it.
Heteronormativity and heterosexism
Heteronormativity denotes or relates to a world view that promotes heterosexuality as the normal or preferred sexual orientation for people to have. It can assign strict gender roles to males and females. The term was popularized by Michael Warner in 1991. Many gender and sexuality scholars argue that compulsory heterosexuality , a continual and repeating reassertion of heterosexual norms, is facet of heterosexism. Compulsory heterosexuality is the idea that female heterosexuality is both assumed and enforced by a patriarchal society. Heterosexuality is then viewed as the natural inclination or obligation by both sexes. Consequently, anyone who differs from the normalcy of heterosexuality is deemed deviant or abhorrent.
Heterosexism is a form of bias or discrimination in favor of opposite-sex sexuality and relationships. It may include an assumption that everyone is heterosexual and may involve a varied level of discrimination against gays, lesbians, bisexuals, heteroflexibles , or transgender individuals.
Straight pride is a slogan that arose in the late 1980s and early 1990s and has been used primarily by social conservative groups as a political stance and strategy.  The term is described as a response to gay pride    adopted by various LGBT groups in the early 1970s or to the accommodations provided to gay pride initiatives.