If there were a gay gene, how would it be passed on if its bearer doesn't want to have sex with members of the opposite sex?
Oh, the gene very well could be passed on and survive if it provided a greater reproductive advantage in the homosexual males' siblings than it cost in missed direct reproduction. For example, if there was a gene which increased the propensity toward homosexuality in men, but, in women, greatly increased fertility or made for healthier babies, the advantage the gene would have when expressed in women would be selected for by natural selection and preserved, notwithstanding the reproductively deleterious effect it has when expressed in men. Even if it man some men gay, it would be preserved if the same gene increased the fertility of those men's sisters. (N.B., I am not saying this is what is actually happening, just presenting a plausible reason why it is not, per se, impossible under natural selection.)
Theoretically that makes sense. I suspect that some combination of genes makes someone a little more or a little less likely to be gay, but it's the cultural/environmental factors that really bring it out.