This begs the question of how we define someone as a lesbian. I’d limit the definition to someone who is only sexually attracted to other women. I’ve noticed that women seem to have a more fluid approach to sexuality than men (no pun intended). I’ve known women who consider themselves straight, but who have had sexual relationships with other women. I’ve also known women who consider themselves lesbians, but who have the occassional relationship with men.
Then that would be a very small group.
Most people won't bother, but if anyone is interested: check out the Kinsey Scale (the Klein Sexual Orientation Grid is better).
When I was in college, they were called LUGs. Lesbians Until Graduation. A psychologist might say something about defense mechanisms or something.
I think that it's just because the Real World is tough place to make a living. You can't just hop down to the local LGBTQ Center for a daily affirmation when someone says something mean to you. That, and basic biology is a mighty powerful instinct.