I don't think so. Nature doesn't seem to limit itself to one particular category in anything, so I doubt it would here.

I think there are heterosexuals, homosexuals, bisexuals, pansexuals, asexuals, and probably some categories I've forgotten to mention. I also think a gay man can sleep with as many women as he wants--he'll still be gay, and not bisexual. I know a couple straight guys who got a little worked up at certain moments with a man, but I don't think that makes them bi necessarily either. Your sexuality isn't defined by who you have sex with.

Nothing is steady and immoveable. Mother Nature evolves and so do you. Some think they're defined as heterosexuals, homosexuals, bisexuals or whatever for good. It takes sometimes a spark of realization (like love at first sight) to light something in your inner mind; an attraction you couldn't imagine. Any event can reveals latent crushes for a gender you wouldn't think of loving beforehand.

I'd agree with that never say never people!

EXACTLY.
Though I seriously doubt my mom will ever come to me to tell me she has fallen in love with a woman.
She had a good female friend of hers fall in love with her, she freaked out and broke off the friendship.

So they say. Personally, the longer I look at a female the odder she looks to me, not stimulating at all no matter how beautiful she's suppose to be, she just starts looking awkward.
[Edited 3/20/09 22:40pm]