Non-lethal mutations are not common, and certainly not commonly recurring.
While not in itself lethal, a gay gene would compel an organism to sex acts that would make reproduction unlikely, and passing along the gene throughout the eons highly improbable.
I find it very unlikely that there exists a gene which will cause both males and females carrying it to tend toward homosexual attractions. I see nothing improbable, however, about the existence of genes which will cause carriers (or either sex) to be unusually attracted to males or which cause carriers to be unusually attracted to females. Since the former type of gene would enhance the reproductive success rates of females and the latter would enhance the reproductive success rates of males, such genes could easily be propagated through millenia, especially since the most likely pairing would be a female with the former gene and a male with the latter. The effect would be that there would be many individuals who had both sets of genes and did not inherit either trait, but who could pass one or the other set of genes to a potential mate without the other.
Note that I believe the genetics would only affect one's 'attractions' to other people. Part of living in society is learning to restrain oneself and not engaging in sexual relations with everyone to whom one is attracted; this applies to heterosexuals as well as homosexuals.