The way evolution is currenty defined, we begin with some random genetic changes. Through the process of natual selection, beneficial traits that appear from these changes will dominate via selective advantage. The animals with these advantages will breed, passing the advantageous genes forward. This strengthens the species.
Homosexuality satisfies the first two conditions, but not the third. It is reasonable that homosexuality is due to genetics. Homosexuality can also create selective advantage under many conditions. But once the gun is loaded, the trigger is not pulled, since step three will require a heterosexual encounter to make a baby so the genes can move to the future.
As an analogy, say the deer are in their breeding season. A large buck with a 12 point rack, wins the mating Olympics. His genes will be passed forward. This will make the herd stronger. But at the last moment, he decides not to breed. The potential for evolution is there, but no genetic cigar. The evolutionary three part cycle of Darwin is aborted at step two.
Even if we assume genetics to begin the process, and selective advantage to make the genes stand out, unless you close the deal in step three, it is like it never happened in evolutonary history. Since homosexual continues to appear, that would mean the same random genwtics appearing again and again at step one. That would suggest this genetic change it is not random but ordered. The idea of genetic ordering conflicts with genetic theory.
It would be like our herd of deer defining their big buck for breeding, season after season. They win selective advantage year after year, but they never breed or want to breed. There is no genetic transmission, yet the big buck comes back every season to win. This suggests new mechanisms for genetics, where genetics can be tweaked into renewable order. I have no problem with that, but current evolution will not hear of it, since order messes up the theory.
If you try to be consistent with evolutionary theory we have the step three paradox. If we explain the paradox with the theory of renewable genetic ordering factors, this offends genetic theory and evolution.