I have heard this theory many times. The primary problem with it is, what would be the mechanism? I have yet to hear a proposed mechanism that makes the least bit of scientific sense. My genes have no idea how many people there are on the face of the earth at the moment. If it is something more local, why is homosexuality so common in the U.S., which is much less densly populated than many countries? One would think that, say, China, India, or Africa would just be hotbeds of all-girl (and all-boy) action.

My theory is that homosexuality, carrying less social stigma than it has in the recent past, is simply becoming more apparent in the first world. It is more common only in that people are more willing to acknowledge their feelings towards MOTSSes---these people would have had homosexual tendencies anyway, but would have suppressed them.

History shows that the societies in which homosexual activity is most prevalent are those in which it is the most socially acceptable. Well, duh. Look at ancient Greece, ancient India (the Kama Sutra discusses oral congress with shampoo boys at great length), etc. It's not just the modern West.