I say for gay men and a rare handful of lesbians, it is genetic. Hardwired. Obvious from a VERY early age - and no child would be able to control their natural impulses -they are what they are.
I doubt any man would deliberately CHOOSE that lifestyle. It's different for women - the emotional issues factor more into it for women than for men. Different needs kind of thing.
But I know many women who made a conscious decision to become a lesbian after abusive relationships of one kind or another with men. They made a choice. No hardwire issues there. I will go so far as to admit that in those very dark days right after my divorce, and a couple of emotionally abusive situations and a string of disastrous blind dates, I considered it briefly - not for the sex angle, but to just have a companion that I could relate to on an emotional level. More like a "BFF" than a "girlfriend"... and if the sex thing was the price to pay, well, fine. But it never came to that, because I literally met Carl three weeks later and the rest is history. Good thing too, because I had no IDEA how to start being a lesbian. Wear a sign? Put an ad in the local GLBT newspaper? Start wearing flannel shirts? Move to wherever it is in south Alabama that is full of lesbians and throw a housewarming party? No. Clue.
And see - no matter HOW bad your relationship lives got, no matter how bad your heart got broken and your mind was swirling, I think it would be a safe bet that NONE of you guys would have ever even come close to thinking about it like I did. Having a guy to hang with all the time, and if he liked a little rear door action every now and again, well, that's the price you pay for love... yeah, RIGHT.