Although I'm not well-versed in genetics, if this were true, wouldn't it be a paradox? Given that homosexual persons usually don't procreate, the gene wouldn't be passed on! When I think about it that way, I can't understand how the theory even came into existence.
I think it is not just one gene that is transferred from parent to child. Rather it is a gene that is being expressed that is usually isn't suppose to be (no offense to gays, 90% of the population are straight, thus the norm).
Also I read somewhere that gays help with population control. If the population is too great then there is a chance that resources will diminish to the point the population suffers and starts to die off. So have a few gays in the mix will help slow population growth.
Homosexuality is a social behavior. It has nothing to do with genetic heritage.
I believe the theory is that homosexuality is supposedly a genetic advantage within tribal groups, as homosexual couples reduce the competition for food (by not having children themselves) and assist in caring for others' children. Thus it could still be selected for.
I believe homosexuality is psychological, just like any sexual preference. Say for instance a straight man has a fetish for women with hairy armpits. He's not born loving women with hairy armpits, but due to various psychological factors during his development, it ends up being something he's attracted to, and there's really nothing he can do about it.