It can be a choice. A lot of women I know, who are lesbians, have had one or two, maybe more bad relationships with men and think 'Hey a woman is more compassionate, I'll be more safe over there'.
They actually have bisexual tendicies, not truly being a lesbian.
Lesbians are like gay men, born that way.
It could make the choice to be with a man, but that doesn't mean I'd like it or ever want it. I was just born with the attraction to females, not men.