It would be so much better if some gay folks would seriously weigh in but I have had some very good friends who are gay, my life once revolved around them for a really long time, so I can give you my opinion/observation based on talking to them, and asking, for myself, as a mostly straight kind of person:
The short answer: People are gay the same way people are straight.
With guys, it seems they aren't as attracted to/comfortable with the opposite sex in an relationship because, generally speaking, they relate TOO MUCH to the opposite sex ... again, my observation. That's why there are "fag hags," chicks who cultivate gay guy friends, you think the gay GUYS don't like hanging with the girls in turn? Woot, be honest, how much do you give a shit about fashion, hair, interior design, shopping? Most gay guys LOVE this shit, really, genuinely LOVE it.
With the girls, honestly, some women really do "go gay" because men are too much of a hassel, have repeatedly hurt them, rape, incest, etc. I have honestly run into just as many lesbians who have had some sort of emotional trauma related to men as lesbians who were "born gay."
What I don't understand is people who are "Asexual." Seriously. I can understand homosexuality, heterosexuality, trisexuality

but NEVER WANTING TO BE WITH ANYBODY, EVER?!?