My answer on Quora:
It is 100% proven that male sexual orientation is fixed by age 15. This comes from studies showing that not only can gays not be turned straight after that age but also gay men cannot increase hetero attraction and decrease homo attraction by that age. So gay men just can’t turn straight, homo attraction cannot be decreased and hetero attraction cannot be increased.
Now we do not know if the opposite is true because no one ever comes to the lab wanting to turn from straight to gay although there is one case written up in the literature. Despite desperate attempts to turn gay over years, a college aged man was not able to do so. He was stuck with women.
But if it works one way it works the other. If gays are stuck by age 15, then straights are too. If gays can’t turn straight then straight men cannot turn gay.
To me this implies that male sexual orientation is determined possibly in the womb, but I cannot prove it. Early childhood experiences have not yet been ruled out.
What we do not know is if male sexual orientation is malleable before age 15.
But yeah, if you are age 15+ and male and cannot figure out your sexual orientation, scientifically this makes no sense. You are fooling yourself or have some anxiety disorder or something is wrong. Sexual orientation is as clear as air for the vast majority of males.