I’m not sure your argument is persuasive. There are plenty of genetic defects that discourage - or, in many cases, prevent - reproduction. Just consider any of the genetic problems that cause miscarriages or severe birth defects. These do not die out, largely because there are carriers who are not themselves afflicted with the condition. Who’s to say the same is not true about a hypothetical homosexual gene?
I am not saying that homosexuality is in any way genetic, only that the continued existence of homosexuals does not conclusively prove that it is not.
Well, in any case, that is only one facet of my argument.