Wouldn't the gay gene have to be passed down through the maternal line?
Any gene that had a 100% chance of causing homosexuality in males would (if we assume homosexual men rarely father children, which is probably true). But even then, if only one quarter of the woman's children would reproduce (only the females and the half of the males who got the mother's non-gay gene), that gene would still die out pretty quickly. Having a 50% disadvantage in reproduction generally causes genes to die out in a short time (it's a 50% disadvantage because the straight son and the daughter without the gay gene wouldn't spread the gay gene, even though they could reproduce, because they wouldn't have it. 3/4 children would be able to reproduce, but only 1/3 reproducing children would carry the gene).
If the "gay gene" was recessive (you need copies from both the mother and the father for it to be active), then it would not suffer a 50% hit to the reproduction rate, but it would still suffer. We would expect that unless it provided some other sort of reproductive benefit (unrelated to causing homosexuality), then it would die out. If having a gay relative helped women to get married and have more kids, then that could be the basis for a recessive "gay gene" staying in the gene pool. If the "gay gene" had some sort of other effect that made carriers more likely to reproduce, it could stay in the gene pool (for instance, if it made carriers smarter or stronger). I'm not saying it's likely, I'm saying it's possible. With science, you have to accept that a lot is POSSIBLE, even if it's not what you WANT to be true.