That's Foucault's theory, but Rictor Norton's book I linked to convincingly debunks Foucault as ideological and ahistorical. Quoting an excerpt, here are historical cases of unmarried men going for each other instead of marriage and children:
In between these two extremes of lust and idealism we find a sense of identity based upon ordinary and unremarkable same-sex love. The records of the Inquisition in Spain, Portugal and Brazil; the police archives of early eighteenth-century Paris; the records of the Officers of the Night of sixteenth-century Venice – all clearly document a preponderance of men who were bachelors and who preferred their own sex. Statistical analysis of the particularly full and detailed Florentine records ‘of the marital status of the men incriminated for sodomy from 1478 to 1483 reveals that fully three-fourths of all such men aged nineteen to seventy were unmarried.
These guys sound like they are exclusive, obligate homosexuals.
As for identity, just because the historical labels for queer people were negative, it does not mean that those terms were just externally-imposed slurs, and that homosexual identities did not exist:
In Foucault’s famous statement: ‘Homosexuality appeared as one of the forms of sexuality when it was transposed from the practice of sodomy onto a kind of superior androgyny, a hermaphroditism of the soul. The sodomite had been a temporary aberration; the homosexual was now a species.’ He ludicrously dates this shift to 1870. But the men discussed in the preceding paragraph had a sense of themselves that transcended both ‘the practice of sodomy’ and ‘temporary aberration’. In fact Dutch sodomites in 1734 were described by contemporaries as ‘hermaphrodites in their minds’ (Boon 1989) – an exact match for Foucault’s ‘hermaphroditism of the soul’. The concepts of masculine homosexual women and effeminate homosexual men dominated the premodern world. The homosexual was considered an androgynous species in Aristophanes, in Juvenal, in all the ancient literature about the transgendered priests of Cybele in the ancient and classical world. It was not a modern construct.
The truth is that a homosexual category existed many centuries prior to the nineteenth century. There are literally scores of fifteenth-century Italian authors who portray homosexual characters rather than homosexual incidents (G. Dall’Orto, ‘Italian Renaissance’, EH), and it is a nonsense to label such sodomites ‘temporary aberrations’ rather than members of a species. In real life there is the famous example of self-labelling, the painter Antonio Bazzi (1477–1549) who was proud of his nickname ‘Il Sodoma’. According to his contemporary Vasari ‘he did not take [it] with annoyance or disdain, but rather gloried in it, making jingles and verses on the subject, which he pleasantly sang to the accompaniment of the lute’.
Rictor Norton is a widely published queer historian, his research goes back centuries, and seems very solid. I think we should go with his account and toss Foucault's social constructionism.
Makes sense. However all these examples are from Christian Western societies, I wonder about non-Western or pre-Christian societies.
Epistemic status: speculating about things I'm not familiar with; hoping to be educated in the comments. This post is a question, not an answer.
ETA: this comment thread seems to be leading towards the best answer so far.
There's a question I've seen many times, most recently in Scott Alexander's recent links thread. This latest variant goes like this:
Obligate male homosexuality greatly harms reproductive fitness. And so, the argument goes, there must be some other selection pressure, one great enough to overcome the drastic effect of not having any children. The comments on that post list several other proposed answers, all of them suggesting a tradeoff vs. a benefit elsewhere: for instance, that it pays to have some proportion of gay men who invest their resources in their nieces and nephews instead of their own children.
But how do we know if this is a valid question - if the situation really needs to be explained at all?
For obvious political and social reasons, it's hard to be sure how many people are homosexual. Note that we are interested only in obligate homosexuality - bisexuals presumably don't have strongly reduced fitness. The Wikipedia article doesn't really distinguish obligate homosexuality from bi-, pan- and even trans-sexuals. The discussion in the SSC comments used an (unsourced?) range of 1%-3%, which seems at least consistent with other sources, so let's run with that.
The rate of major birth defects in the US, as reported by the CDC, is also about 3%. This counts both developmental and genetic problems, and includes everything from anencephaly (invariably fatal) through Down syndrome (severe but survivable) to cleft palates (minor). But most of these, at least 1.5% of births, were always fatal before modern medicine, and many of the others reduced fitness (via mate selection, if nothing else). Various other defects and diseases, which only manifest later in life, are also thought to be influenced or determined during early development. And so is sexual preference.
(Whether homosexuality is a developmental disorder is not the point; I'm comparing the effect of selection pressure on fatal teratology with its effect on reduced-fitness homosexuality.)
Embryological development is a complex and fragile process, and there are many ways for it to go wrong. We don't wonder how it is possible that selection pressure allows anencephaly to occur in 1 in 4859 births. There are certainly direct causes of anencephaly, explanations of why it happens when it does, but (I think) we don't a priori expect them to be due to tradeoffs yielding benefits elsewhere. It's just as plausible that the tradeoffs involved are against even worse (counterfactual) problems elsewhere - or that there are just no available mutations that don't have these or equally severe problems.
Could it be that linking sexual preference to the biological gender is, for some complex developmental reason, fragile enough that it goes wrong despite all selection pressure to the contrary, that it has no redeeming qualities from the viewpoint of evolution, and that is all there is to it?
When faced with any phenotype with reduced fitness, how can we judge if there is something to be explained - a beneficial tradeoff elsewhere to search for - or merely a hard problem evolution couldn't solve completely? And is there a way to quantify this question, relating it to the known mathematical models of genetics?
Notes:
1. I'm posting this in the spirit of recent suggestions to post more and accept lower quality of (our own) posts to Discussion.
2. I'm going to sleep now and will start replying to comments about 10 hours from now; sorry for the inconvenience.