Editing Heterosexuality (section)

=== Biological and environmental===
{{Main|Biology and sexual orientation|Environment and sexual orientation}}
No simple and singular determinant for [[sexual orientation]] has been conclusively demonstrated, but scientists believe that a combination of genetic, hormonal, and environmental factors determine sexual orientation.<ref name="pediatrics2004" /><ref name="Lamanna"/><ref name="Stuart"/> They favor biological theories for explaining the causes of sexual orientation,<ref name=Bailey16/><ref name="pediatrics2004"/> as there is considerably more evidence supporting nonsocial, biological causes than social ones, especially for males.<ref name=Bailey16/><ref name=LeVay/><ref name=Balthazart/>

Factors related to the development of a heterosexual orientation include [[gene]]s, [[Prenatal hormones and sexual orientation|prenatal hormones]], and [[Human brain|brain]] structure, and their interaction with the environment.

==== Prenatal hormones ====
{{Main|Prenatal hormones and sexual orientation|Neuroscience and sexual orientation}}
{{multiple image
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 | width1 = 168
 | image1 = 2AM9.png
 | caption1 = Structure of the [[androgen receptor]] (rainbow cartoon) complexed with testosterone (white sticks).<ref name="pmid16641486">{{PDB|2AM9}}; {{cite journal |vauthors=Pereira de Jésus-Tran K, Côté PL, Cantin L, Blanchet J, Labrie F, Breton R | title = Comparison of crystal structures of human androgen receptor ligand-binding domain complexed with various agonists reveals molecular determinants responsible for binding affinity | journal = Protein Sci. | volume = 15 | issue = 5 | pages = 987–99 | date =May 2006 | pmid = 16641486 | pmc = 2242507 | doi = 10.1110/ps.051905906 }}</ref>
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 | image2 = Testosterone-from-xtal-3D-balls.png
 | caption2 = [[Testosterone]] contributes to the masculinization of the brain
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 | image3 = Oestradiol-3D-balls.png
 | caption3 = [[Estradiol]] also stimulates the [[androgen receptor]]s.
 }}

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 receptor]]s 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.<ref>Vilain, E. (2000). Genetics of Sexual Development. Annual Review of Sex Research, 11:1–25</ref> It has been suggested that both male and female heterosexuality are the results of this process.<ref>Wilson, G. and Rahman, Q., (2005). Born Gay. Chapter 5. London: Peter Owen Publishers</ref> 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.

==== Animals and reproduction ====
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 | image1 = Bonobo sexual behavior 1.jpg
 | caption1 = Bonobos mating, [[Jacksonville Zoo and Gardens]].
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 | image2 = Hoverflies mating midair.jpg
 | caption2 = [[Hoverfly|Hoverflies]] mating in midair flight.
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{{Main|Animal sexual behaviour|Mating}}
{{See also|Pair bond|Mating call}}

Sexual reproduction in the [[animalia|animal world]] is facilitated through opposite-sex sexual activity, although there are also animals that [[Asexual reproduction|reproduce asexually]], including [[protozoa]] and lower [[invertebrates]].<ref>''The Columbia Encyclopedia'' (Colum. Univ. Press, 5th ed. <nowiki>[</nowiki>casebound?<nowiki>]</nowiki> 1993 ({{ISBN|0-395-62438-X}})), entry ''Reproduction''.</ref>

Reproductive sex does not require a heterosexual orientation, since sexual orientation typically refers to a long-term enduring pattern of sexual and emotional attraction leading often to long-term social bonding, while reproduction requires as little as a single act of [[Copulation (zoology)|copulation]] to [[Fertilisation|fertilize]] the [[ovum]] by [[sperm]].<ref>{{cite web|url=http://www.goaskalice.columbia.edu/0116.html |title=Go Ask Alice!: Pregnant without intercourse? |access-date=January 24, 2016 |url-status=dead |archive-url=https://web.archive.org/web/20111222043503/http://www.goaskalice.columbia.edu/0116.html |archive-date=December 22, 2011 }}</ref><ref>{{cite web|url=http://www.americanpregnancy.org/preventingpregnancy/pregnancyfaqmyths.html |title=Can Pregnancy Occur &#124; Pregnancy Myths on How Pregnancy Occurs |website=Americanpregnancy.org |date= 2012-04-24|access-date=2016-07-07}}</ref><ref>Lawyers Guide to Forensic Medicine SBN 978-1-85941-159-9 By Bernard Knight - Page 188 "Pregnancy is well known to occur from such external ejaculation ..."</ref>