[CORed]

Another possibility is that a recessive "gay gene" confers some sort of survival advantage in heterozygous individuals. What that might be I have no idea (better fashion sense gives males with one copy of gay gene a better chance of getting laid? :-) ).

This would be analogous to sickle cell anemia and cystic fibrosis, both lethal recessives. However, people with just one copy of the sickle cell gene have increased resistance to malaria, and people with one copy of the cystic fibrosis gene have better resistance to water-borne diarrheal diseases (leading cause of infant and child mortality where sanitation is bad, which was just about everywhere until 100 years or so ago).

Actually, I suspect that the reality is much more complex. There is probably a gene, or more likely several genes, that create a pre-disposition to homosexuality, but there are probably psychological factors that trigger it, as previous poster suggested. There still may be some sort of survival advantage to the gene or genes. Non-genetic developmental effects could be a factor, too, such as hormone levels in the mother. It seems to me that I have read of studies of identical twins, and there was something like a 50% concordance rate. If it was purely genetic, it should be 100%. Of course, the true number may well be higher, due to one twin being in the closet and one openly gay. Also, twin studies don't control for psychological factors, since ususally both twins grew up in the same home. Finding enough gay identical twins separated at birth to make a significant sample could be difficult, though.

And of course, it is correct that gay vs. straight is not a black-and-white matter. Many straight people have gay sex at some time in their lives, and vice versa. Sometimes the heterosexual encounters by "gay" people produce children. Also, quite a few lesbian women have children with donor sperm, sometimes using gay men as the donors, so I don't think homosexuality is going to be completely selected out any time soon.