Reddit mentions of Homophobias: Lust and Loathing across Time and Space

It sounds a bit more like this question is being phrased in an evo-psych way rather than an anthropological way, really. Since others have address this issue more than adequately, brigantus in particular, I won't say anything else about it.

But since you're curious about the intersection of anthropology and neuroscience, you might be interested in the sub-field of neuroanthropology. Greg Downey and Daniel Lende have an aptly titled blog, neuroanthropology, over at PLOS, as well as a book on the subject. As far as I know they don't really address homophobia specifically, though I haven't had the chance to read it.

Anyway, if it isn't too off topic, it may help to briefly talk about how anthropologists have actually tried to talk about homophobia. What does an (socio)anthropological approach to homophobia actually look like?

A good book on the subject is David Murray's edited volume Homophobias. It raises the issues that come up with actually treating homophobia as an ethnographic and anthropological subject (how do we define it? how do we study it?), as well as providing a variety of ethnographic examples. There's some meaningful difference, I think, between the kind of homophobia that Sullivan-Blum talks about in her chapter on evanglical Christian opposition to gay marriage - where gay marriage isn't so much defined as disgusting but more of an epistemic threat - and the development of a violent/potentially violent homophobic politics in Indonesia and Jamaica, as detailed by Tom Boellstorff and Suzanne LaFont respectively.

I'd also recommend looking at Gayle Rubin's work; specifically "The Traffic in Women: Notes on the 'Political Economy' of Sex". In "The Traffic in Women" she lays out an argument for the social origins of obligatory heterosexuality (and therefore, homophobia) by combining Levi-Strauss, Freud, and Marx.

In contrast to a bio-evolutionary account of heterosexuality/homophobia, she points out that "if biological and hormonal imperatives were as overwhelming as popular mythology would have them, it would hardly be necessary to insure heterosexual unions by means of economic interdependency."

Instead, as firedrops also points out, we can better understand gender identity and sexual desire (and arguably also sexual disgust) by understanding them as socially constructed; as social adaptations to the political-economic problem of how society should be organized/reproduced, rather than purely neurological artifacts of our evolutionary history.

Rubin writes that:

"The division of labor by sex can therefore be seen as a "taboo"; a taboo against the sameness of men and women, a taboo dividing the sexes into two mutually exclusive categories, a taboo that exacerbates the biological difference between the sexes and thereby creates gender. The division of labor can also be seen as a taboo against sexual arrangements other than those containing at least one man and one woman, thereby enjoining heterosexual marriage."

and

"Far from being an expression of natural differences, exclusive gender identity is the suppression of natural similarities. It requires repression; in men, of whatever is the local version of "feminine" traits; in women, whatever is the local version of "masculine" traits."

In 'traditional' (yes, not the best term) societies, opposition to female homosexuality (where such opposition exists) can be best understood by pointing to Levi-Strauss' notion that marriage is the exchange of women by men. 'Homophobia'/obligatory heterosexuality as opposition to lesbianism, then, would be due to the fact that "if a single refusal were disruptive, a double refusal would be insurrectionary." Obligatory heterosexuality, expressed as homophobia against male homosexuality, could be understood as part of that repression of femininity in men - an argument that R.W. Connell takes up in her book, Masculinities.

Anyway, hopefully that was somewhat readable and didn't meander too much.