I don’t believe we ever really thought that homosexuality was ever decided simply by your genes. Identical twins, siblings, etc, would all tend to discount that.
BUT, there is some fairly good evidence (I apologize for lack of cites, I learned this in Psych class) that there are some genes that would increase the likelyhood of one being homosexual. It’s then up to other factors to select homosexuality (the person’s opinion has nothing to do with it, I’m meaning things like biological, really early childhood stuff, etc). Genes simply are one of the many factors.
I explicitly brought up the idea of a gene complex and of recessive genes. You’re aware of the gene that in heterozygous individuals provides partial protection against malaria and in homozygous individuals causes sickle-cell anemia? And is therefore selected for as well as against?
Along the same lines, purely as a hypothetical example of the sort of causal sequence that could select for a gay gene, think of the two archetypes of the loner and the two loyal comrades. Assume that these personality types are based on the absence or presence of a gene coding for male bonding. The loner is homozygous for the absence-of-male-bonding gene, the two comrades are heterozygous with the bonding gene being expressed. Now assume that homozygosity for the bonding gene results in sexual orientation towards the comrade. In almost any culture, while there is a place for the loner, the most effective individuals are those who can team up and work together, done through male bonding. So there’s a strong force selecting for that gene – yet its doubled expression causes gayness.
I don’t seriously propose this as “the gay gene” (which I suspect is mythical, other than being the presence of something that in some circumstances may cause a predisposition). But it’s one explanation. The idea of men who, being gay, were unencumbered by wife and children being of value to early human cultures, which tended to survive where the cultures where all men had wives and children to defend did not, is another serious argument for how a gay gene might be selected for.