Two thoughts. First, genetic mutation/problems/diseases occur all the time throughout nature. These haven't "died out" over history.
Second, a personal observation. I grew up knowing a pair of male twins. One was kinda girlish (e.g. liked to play with dolls), the other more masculine (more interested in sports and all). I think there's a genetic component to gay behavior - not the only explanation - but part of the puzzle.
Are these twins "Identical Twins" or "Fraternal Twins"?
That's where I am with it.
True. Genetic mutation/problems/diseases do pop up. Let's take something similar -- alcoholism. It is also questioned as to having a genetic component along with an environmental cause. Plus, you can do your same identical/fraternal twin studies.
If alcoholics did not breed, therefore, there would be even less occurrences than there are now. So, eliminating the possible genetic component completely will not stop the mutation, only reduce the occurrence.
Alcoholism, like homosexuality or pedophilia, may be "genetic" in the sense that a person, randomly, may be predisposed to that behavior. That doesn't mean that you have to drink -- one can decide to quit.
Other genetic mutation/problems/diseases aren't rooted in the rejection of heterosexual sex, which sends genes up the line.