It isn't in their genes. It's the same reason there are some "pre-primed" for PTSD and come home messed up after being involved in some crazy things while others aren't. They have pre-existing trauma. In this case it may be generic trauma, but often sexual trauma, and definitely in life, they've been pushed to an out-group. When you're a kid and don't belong, you ask why. When the answer is that people who don't belong might be gay or trans (or have alters like the current evolution of this), it becomes an option but also an answer to some who are looking for a cause.