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Heterosexuality describes a state whereby an individual is primarily attracted to members of one or more sexes and/or genders that are not their own, and not attracted to sexes and/or genders that are their own. Statistically, the vast majority of human beings at any one point in history are heterosexual.
A slang term, generally but not exclusively used by homosexuals to refer to heterosexuals is 'breeders'. Heterosexuals can also be described as 'straight', and, by small-minded bigots, as 'normal'.
Causes of heterosexuality
Heterosexuality is caused by the exact same things that cause any other sexuality, and heterosexuals are just are powerless to change who they are as anyone else is.
Were we inclined to mirror right-wing discourse on homosexuality, we might say something like, 'The causes of heterosexuality are to date largely unknown. Some have argued that so-called 'straight' people were born that way, whereas other posit that heterosexuality is a choice. Furthermore, some scientists argue that heterosexuality is a result of a variety of factors, both biological and environmental.' but of course this is dehumanizing and laughably ignorant, just like it is when they do it (and they do it a lot more).
Implications of heterosexuality
It is notable that heterosexual partnerships constitute the single largest amount of divorces nationwide, along with rapes and incidents of domestic violence, though this largely stems from there simply being more heterosexual people than homosexual, bisexual, asexual, or pansexual people. Once you adjust for differences in population size, relations of any orientation are approximately as stable as any other - but that makes for less inflammatory discourse than simply baiting people with "most relationship problems happen in straight relationships!", which is technically correct, if misleading.
It is common for people to say that a world without straight people would swiftly see humanity go extinct, and by extension, that heterosexuality is what keeps humanity going. Strictly speaking, this is not correct, at least in the modern age, as sperm banks and artificial insemination, and in vitro fertilization all allow for impregnation without heterosexual intercourse - though it would be impractical to suggest that such measures could be applied to every human being on earth right now, as the technology simply is not universally available. Some sort of force preventing heterosexual intercourse would lead to a rapid and drastic population decline, though the aforementioned measures would keep our species from total extinction.