If homosexuality is influenced by genetic factors, then why aren't those genes selected against, and why haven't they been eliminated from the population? A recent post by Quintessence of Dust answers that question, and relates it to Cystic Fibrosis, which, in addition to making people very sick, usually renders them unable to have biological children. CF is inherited, and, of course, it has not been removed from the human population by selection. Heterozygote advantage is well known, and has been, in some genetic systems, like sickle-cell anemia, for decades. Sickle-cell anemia remains in populations in spite of the severity of the homozygous recessive condition. The inheritance of sexual orientation is apparently much more complex than that of CF or sickle cell anemia, but similar mechanisms may be active in maintaining genes for homosexual behavior in the human population.
In sum, genetic causation of homosexuality is possible, even assuming that homosexuals never had offspring to pass on their genes (which is an unrealistic assumption).
A couple of related matters: 1) Homosexual-type behavior has been observed in many animal species.
2) In this post, one of mine that has been most frequently read, I attempt to discuss the matter of homosexuality in general, as related to what the Bible says.
Thanks for reading.