OK: so what follow are the premises of my thought-experiment. They are themselves relatively evidence-free, and may be wrong. But they strike me as within the bounds of plausibility. The starting point, upon which everything that follows, is the dodgiest (from what I read, there is some evidence supporting it, but it may be completely untrue and it is almost certainly much more complicated than any simple account might suggest). It's this: homosexuals inherit their desire to have sex with people of the same gender as themselves. By that I mean: homosexuals do not consciously 'choose' to be gay, and equally cannot simply 'choose' to be straight. But I mean something more than that: I mean that, to the extent that homosexuality constitutes a necessary component of who any particular gay man or women is, it is part of their genetic makeup.
Now, here's a second key premise. Some gay people want to have kids of their own, of course; and some gay people go on to do precisely that. But: looked at overall, gay people are less likely to pass on their genes than straight people. The whole system of society is set-up to facilitate straight people having and raising kids, after all, and is often explicitly hostile to gay people doing so even when homosexuality is ostensibly decriminalised. Plus gay men and women must, by and large, make conscious choices to step away from their usual sexual practice in order to conceive kids. Accordingly, straight people are more likely to have kids than gay people -- not that gay people can't, and certainly not that they shouldn't, but that, broadly speaking, they don't.
Which leads me to premise number three. If 'gayness' is a heritable quality, then we might expect two large-enough populations of breeders, one straight and one gay, to produce offspring that are weighted, respectively, accordingly: with more straight orientations in the former case, and more gay in the latter. Of course it's not true that gay people will inevitably breed gay children, or that straight people will inevitably have straight kids -- very often, empirically, that isn't the case; and we know that that's not how genetics work (as our own experience confirms; for our own children are not simple clones of ourselves). But over time, and broadly speaking, we might start to see a diverging trend between these two populations. Because that is how genetics works: on large populations, over long-enough periods of time, to magnify traits.
Now, here's a 'what-if?' What if a global climate in which gay people can live 'out' lives, decriminalised and without stigma, eventually begins to breed-out homosexuality from the population?
This is to hypothesise one key argument about human social history. Until recently, large numbers of gay men and women were in the position of being compelled, by oppression and criminalisation, to live 'straight' lives. On an individual level, it is clearly an ethical monstrosity to create a climate that forces people to live a lie. But could it be the case that this broader climate kept the same proportion of gay people in the business of producing children as was the case for straight people? (ie: not all straight people have kids, any more than all gay people do -- but when gayness was criminalised, these two proportions were closer, and now that gayness has been decriminalised it will tend to move apart). Or to put it another way: in previous generations, many gay people were effectively pressganged into heteronormative roles in their culture that they would not have chosen to fill, had they had a choice; and amongst those roles was the expectation that they would have kids.
To model it, crudely: let's say a combination of genetic factors and social pressure to pretend to be straight result in 4% of the population being gay; but let's also say that the transmission of these genes depends upon gay men and women having kids that they would not, if given the choice to live 'out', free lives, have had. Now imagine a world in which that social pressure was removed, and gay people only had kids if they actively chose it. The obvious extrapolation would be -- a smaller number of gay people will have kids, and the proportion of the population being gay will diminish. The asymptote (or doomsday scenario) here would be: the proportion of the general population that is gay falls into statistical nullitude.
If there's anything in this (and of course I concede it's a big if), does it have any larger ethical consequences? To thumbnail it: most people, or most people outside the centres of world religious fundamentalism and bigotry, would agree that the ethical way to behave is to let people live their lives according to their own sense of their sexual orientation. But what if, on a larger scale, and across the generations, this results in homosexuality becoming, relatively, much rarer in society?
It's possible to turn this around: there are relatively powerful activists, particularly in the Bible Belt of the USA, who simply refuse to accept gayness as one fact of human life. What these people want, it seems to me, is to go back to a time when gay people were forced to live the heterosexual lie. But what if, by doing so, these bigots were actually guaranteeing the continuing presence of a sizeable homosexual contingent in society as a whole?
My gut feeling is that if (big if) a worldwide, longlasting culture of tolerance leads to a dimunition of the proportion of the population -- then that would be a bad thing, broadly speaking. But, I wonder why I think so? Certainly, I don't see any reason to argue that gay people ought to be driven underground in order to 'preserve' gayness as an aspect of human culture; the trade-off (enormous human misery now for some nebulous future 'gain') is simply untenable. But I do wonder.
Who else has argued-through this thought-experiment, I wonder?