It's a fair question, homosexual genes and human evolution aren't necessarily at odds with one another, for the simple fact that even in the modern world, a great many homosexuals still have children. I think most of us likely know somebody directly, or know of somebody, who grew up traditionally, had a traditional marriage/kids due to social pressure, then later in life chose to embrace their innate attractions. That happens often in our society where homosexual lifestyles are generally accepted (to a degree). Now rewind through the history of civilization to a time when they would be murdered off hand for that lifestyle and it's easy to see how the genes could have survived.
What makes the whole thing extremely interesting, is that it is highly likely that the majority's distaste for homosexuals is probably also genetic. It makes sense that our paleolithic ancestors likely shunned homosexuality (a group that embraced it would be unlikely to survive) more so than most do now. Pushing many people into the prehistoric closet, (or the cave as it were).
The theory also holds up due the fact that just about every major religion is anti-homosexual, this goes hand in hand with the reality that our social theories and morals come from evolutionary inclinations toward group survival.
So because it is in our genes to have a distaste for homosexuality, is it then okay to marginalize this group of people? Hardly, the whole point of civilization is for us to apply our rational brains and overcome evolutionary tendencies. We no longer practice infanticide if we feel that a new baby will stretch our resources too thin, or the child is too sickly, we no longer begin breeding our daughters as soon as they reach fertility, the list goes on and on.
The world continues to move on for the better.
side note: I really don't get the "homosexuality as a sin" logic. Typically "sins" are attributed to desires that we all hold, lust, envy, greed, etc. How can something be a perverse and sinful choice if it's not something that most people are ever tempted to do? In fact, something that straight people find to be outright gross . It doesn't wash, give it up, the argument is pretty much over. Nobody can come up with a single argument against homosexual acceptance other than "the bible says...." Well fine, but thankfully we don't make laws and social order based on the teachings of bronze age holy books. There is a group that's trying to get that kind of thing going in the middle east, seems like a real hoot!