[BortimusPrime]

My guess is that it's ultimately a no-cost mutation linked to something beneficial. Genes don't operate in isolation, for example blacks getting sickle-cell is connected to malaria resistance. The penalty of homosexuality assumes that gays are incapable of producing children, but in a traditional society where women are married off like cattle and men associate their status with maintaining a family you would have incentive for gays to breed while occasionally popping out to the bath house for a little "philosophy" with some other ancient greek dudes. The benefit may be that the genes causing gayness to a certain extent make the person more attractive to the opposite sex. I've seen way more gay guys that manipulate their female friends easily, compared to the screeching Nathan Lane types. Same with lesbianism, at the cost of a few disgusting bulldykes you get bi girls willing to do threesomes.