Are you sure about that?
First of all, I'm confident there is no such gene. But, if there were, it wouldn't necessarily be eliminated by "evolution" (or survival of the fittest).
Think recessive vs. dominant, as with blue eyes vs. brown eyes. I'm no expert, but I know there are genetically-transmitted abnormalities which are so recessive that only a rare genetic combination will cause the abnormality to appear, when both seemingly-normal parents (as well as the siblings, and some or all of the grandparents, etc.) are carriers.
The miniscule genetic advantage of having brown eyes over blue is one thing; the advantage of having a reproductive drive vs. not having such a drive is entirely different. If there were a gay gene, how would it be passed on if its bearer doesn't want to have sex with members of the opposite sex?
I guess there are things like "linked traits" and so on that make it theoretically possible. But I find it very unlikely.