Here's the first link from Jason (more generally about how certain sexually transmitted diseases might affect behaviour), and then you go to Frost's blog, and follow his series of posts on the topic.
It seems to me that the idea of Frost becomes unreliable at the first step, where he argues that it was only from the late 19th to early 20th century that there arose a lot of exclusively homosexual men. Sure, the argument goes, there has always been homosexual behaviour, but:
In the Middle Ages, this behavior was seen as a ‘vice’ of older heterosexual men, typically with young boys or men of a servile status. In contrast, far fewer men were exclusively homosexual in the sense of being uninterested in women and resembling women in their sexual orientation (i.e., having a woman’s search image and desired self-image). This relative rarity is implied by the astonishment that European explorers felt on encountering Amerindian berdaches during the 18th and early 19th centuries (Désy, 1978).Now I know Foucault argued that the idea of "the homosexual" is a modern construct (basically, a fundamental change in the way people understood sexuality and themselves), and I have written before that much of his theory sounds plausible; but this "gay virus" idea (at least as adopted by Foster) takes it a step further, and suggests there is a biological reason as to why people started experiencing sexuality differently.
Surely, this is an improbable step too far. For example, isn't it very likely that the matter of "third sex" categories turning up in quite a few indigenous societies must have encompassed not just what we would now call transgender individuals, but at least some exclusive homosexuals?
And what of the difficulty of judging the experience of homosexuality based on historical records? Just because most of the contemporary commentary we have of homosexual behaviour in a society might be about (say) the pederasty of older married men (see Greece), it may simply never have occurred to anyone to research carefully what men as a group felt about their own experience of sexuality over a lifetime. I mean, it's even controversial now, the scientific studies of bisexuality that involve trying to work out what gets certain men subconsciously aroused. (There was a good article about this earlier this year in the New York Times. It seems that a study a few years back which got widely reported as showing that bisexual men really did respond more as homosexual is now accepted as having a particularly poor experimental set up.)
The point is this - what hope do we really have of a detailed understanding of subjective experience of sexuality across a population of men from 2000 years ago, before anyone really had ideas of objective, scientific surveys or any other detailed form of psychological research?
Foster is especially unconvincing when he argues that the "flapper" fashion of the early 20th century - where women strove to look more boyish and flattened breasts were in vogue - indicates anything meaningful about the heterosexual male "search image". Doesn't the advent of Marilyn Monroe and other famous, buxom stars of a mere 30 years later put a serious dent in this suggestion?
We certainly know there has always been a lot of concern in Abrahamic religions through the centuries as to how much homosexual behaviour was going on. The article "The Experience of Homosexuality Across the Middle Ages" gives a good idea of that. (And interestingly, its author now says in an updated preface that he has changed his mind about many of the paper's conclusions. I don't really know what he believes now.) My point would be: who knows whether a monastery which had a particular reputation as a haven for homosexual activity in 1500 CE was that way because it had attracted exclusive homosexuals, or because of jail-like opportunistic homosexual behaviour, or in what proportion it was a case of one and the other? Bishops and saints didn't really care to psychologically investigate it before condemning it.
In any event, not that I have made any detailed study, but it still seems to me that no one really understands the interaction between gender identity, sexual identity and sexual experience. Sure, there is strong reason to believe the widely reported phenomena of a gay man who has "always known" he was different from other boys - but why does the difference sometimes manifest in conviction that they are the wrong gender, not just attracted to their own gender?
It's something of an understatement to observe that the brain and mind are complicated. For example, I find anorexia as a psychological disorder incredibly hard to fathom - how it is that the rational part of the mind cannot convince the other part of something that I would have thought obvious from looking in the mirror, along the lines of "OK, this is really getting unhealthy now, you look like skin and bones and as the doctor says, you're going to die unless you start getting food into your body." I have a similar attitude towards late onset transgender cases - come on, fellas, you've been married, climbed mountains, had kids and presumably quite a few orgasms with your wife. Why is changing gender now going to be some kind of fulfilment? Can't you just take up quilting, or something a bit less dramatic?
In any event, history at least teaches us that varieties of same sex activity have always had some appeal to some people, and anthropology also indicates that people who want to be the other gender have always been with us . It seems that in some societies, homosexual behaviour was treated as a mere matter of taste and not the subject of much in the way of analysis. So much of the controversy about it since then seems to have come from attempted overanalysis which may well be forever doomed to be an example of one of the suggested implications of Godel's incompleteness theorem, that the mind may never fully understand itself. See this quote from Hofstadter:
The other metaphorical analogue to Gödel’s Theorem which I find provocative suggests that ultimately, we cannot understand our own mind/brains … Just as we cannot see our faces with our own eyes, is it not inconceivable to expect that we cannot mirror our complete mental structures in the symbols which carry them out? All the limitative theorems of mathematics and the theory of computation suggest that once the ability to represent your own structure has reached a certain critical point, that is the kiss of death: it guarantees that you can never represent yourself totally.
No wonder the current Synod is fretting about what to do, if (as I suspect) our whole understanding of sexuality, being an aspect of mind and personality, is always likely to be incomplete.
Update: on re-reading this post, I think I need to clarify my position, because it perhaps got lost a bit in the ramble:
a. it may be that a lot more men in the 20th century have started thinking of themselves as exclusively homosexual, but the reason for this is most likely the Foucaultian explanation about the change in thinking about sexuality - broadly, along Freudian lines that there is a large hidden element to it that people sooner or later uncover.
b. But as to whether the number of pretty much exclusively practicing homosexual males has changed over history is very much guesswork, given the lack of any detailed interest in anyone until about the 20th century in trying to work out such numbers.
c. Even current thinking, of what we think to be enlightened modern thinking on sexuality, is very much a work in progress, and one in which people seem always too sure of themselves that they understand it and its intersection with gender, sexual experience and culture.