What you seem to be suggesting is that gay and straight are phenotypes that vary in a polymorphic way (in the simplest sense). I don't have any clue if that's possible and I really doubt that's what's going on but it's an interesting hypothesis.
I fail to see why evolution would at any point "do things" that would result in less propagation of the species. I think creepyinfant 's theory attributes a lot more intelligence to evolution than there actually is. Evolution is not smart. Evolution does not say "We need less people" and then make there be less people. Evolution is determined mainly by traits that encourage sexual success and propagation. Those traits can vary depending on circumstances. However, it seems very unlikely that we would evolve towards not propagating. I think b_b and theadvancedapes are better placed to speak on this than I am, however.
Well, we need to keep in perspective the timelines in which species evolve, and compare that to the timeline of human society. Homo sapiens has been a thing for roughly 500,000 years, civilization has been a thing for 10,000 years, and homosexuality as a conceptual construction has been a thing for maybe a century. Men have been banging men for a very long time, as we all know, but a Spartan who fucked a Spartan while marching toward some foreign war still eventually went home to his wife and made babies. Hence, even if there is a such thing as a gene "for" being gay, it wouldn't have started to work its way out of the genome until one could be gay, or you know, since the 70's. There are a lot of examples of homosexual sex in nature (elephants and bonobos, for example), but there doesn't (as far as I know) appear to be gay behavior (that is, exclusively favoring one's own sex, but I'm no behaviorist; I'd love to hear if anyone knows an example of this). I think homosexuality, as we currently know it, is a by product of society more so than a genetic aberration (although, as with most traits, I'm sure some are more genetically predisposed to such behavior when exposed to certain environments). So the short answer, is that if there is a gene for being gay, then it has only recently started to be whittled out of the genome, and for it to really be worked out it would require everyone who feels a little bit gay to stop breeding altogether. Not very likely.
You're right of course none of us know what we're talking about compared to the hubskiers who study this but I'd rather muddle about and form some intelligent questions to ask them once they come along instead of just learning the boring way.I fail to see why evolution would at any point "do things" that would result in less propagation of the species.
He posits overcrowding. That would have a selective effect automatically, right? But I don't think has anything to do with sexual preference.