I have a sense that these writeups may have overlooked the obvious: what if the mechanism is psychological rather than genetic, and occurs with socialization, rather than in utero? High population leads inevitably to resource scarcity, which in turn leads to greater stress on individuals, whether by going hungry or living in an industrial society. That stress will affect all other aspects of a society, possibly even sexual orientation. There being no alternative but to return to more primitive conditions, I'd have to say all's right with the world.