And the biologists do believe that "altruism" has a genetic component. Your arguments about the recessive gene gradually being eliminated would be equally applicable to that. So you would think they would be aware of the argument.
This is also true for recessive genes that produce albinism. Yet albinism is still with us. And note that in most cases, neither the parents nor the grandparents nor the great grandparents of a person with albinism manifest the condition. Yet it continues to be passed along. So...it is more complicated than your argument makes it appear.
Perhaps, even after all these years, a rare trait can still exist by carrier match-ups only. But gays claim 10% of the population.
Also, how about other sexual behavior then -- whether "good" or "bad?" Can anyone be held responsible for anything?
I do not even buy the altruism argument. How does "doing things for the benefit of others" apply? Are you saying homosexuality benefits others and is some kind of act of sacrifice? I really don't get that.