This is an older thread, but I came across it and wanted to add a few things.

First off, I'm Bisexual. The way I knew, for sure, was that I am not only physically attracted to both men and women, but I can also be romantically and emotionally attracted to both genders. I can have a connection with either sex. Some people say that if you're truly gay or bisexual, that you have to be able to picture yourself in a relationship with a same sex partner - not just having sex with one. I don't know if I always agree with that, but for me it rings true.

I would also rather watch two women together than a man and a woman together, there is just something a lot more sensual about two women.

Sexuality can be very fluid, and there isn't always an answer to this question. My best advice would be to just "go with the flow." If you meet a woman that you're attracted to, and feel inclined to take it a little past friendship, then go for it! If you meet a man that you're happy being with, that's fine too! Don't put so much pressure on yourself to identify with one particular label...just be yourself, and do what feels right in your heart.