First of all, there is no gay gene so far. They're been looking for it for decades and nothing was found, not even in a recent gigantic genetic study (500k people).
The problem is that **you can not prove a negative**. They will always cling to the possibility of such a gene existing and you CAN'T contradict them.
However, even if tomorrow they find the "gay genes", it still means nothing in regards to LGBT not being a disorder.
You have genetic heart problems, genetic liver problems, genetic kidney problems and i could go on and on. If such a genetic disorder affects a person we don't say "well, he was BORN THIS WAY", so let's leave it like that, it's normal. We acknowledge it's a disorder and we try our best to fix it.
Every time you engage with some degenerate in the "homos are genetic - no they're not", you're only enabling and empowering their position while crashing yours. The only answer, which you should repeat ad nauseam, is that **we have thousands of disorders and we don't let them be because they are genetic, we fix them**.