How would such a gene propagate?
For millions of years, you are saying that a gene that was not conducive to heterosexual propagation somehow managed to remain in the population?
It's not a survivable gene. Maybe if humans had hermaphroditic reproduction, but...we don't. And artificial insemination is an exceedingly recent invention.
It's not genetic. There may be a predisposition based on hormonal production, but it would eventuate only under the right conditions, and then die out....because there would be no offspring. Or very few, in some unusual cases.
Not if as for most circumstances because homosexual behavior is frowned upon by most societies they may not be open about it and reproducing in higher numbers than you think. We simply don't know or have enough data. Political correctness has made research in this area a third rail for researchers and the fact is we have few clues on the subject.