One thing I would really recommend to people who haven't fully established their orientation is to give some serious thought to this whole subject of labels (gay, bi, straight, etc.). I would guess that most of us don't fit neatly into any particular category. We're human, and human sexuality is an extremely complex thing, and it works on many different levels.
It seems that we like to label ourselves because it helps us establish our identity. The label I choose to define my orientation helps me to establish who I am. It also conveys information about me to others who might want to know what kind of relationship they might have with me - platonic or romantic.
I personally can enjoy basic mechanical meaningless sex with men, though I prefer women. Technically, I'm bi. I have felt strong desire for a few men - and many women. The emotional bond, the electric chemistry, the intense desire, longing, passion, etc. - for me, that happens only with women.
And that brings up a HUGELY important point: it isn't just about sex. A genuine relationship is much more than that, and operates on so many levels.
I have found it difficult to get any interest from lesbians if I tell them I'm bi. I have found there to be an anti-bi bias in the gay community. And I understand it. I know a lot of gays have been burned by bis who choose the easy way out - the straight life. They love to point out the whole Anne Hecht leaving Ellen for a man scenario as an example.
So - I have found it easier to label myself a lesbian. Much easier to get dates, and I suppose it's not really dishonest since I really want a relationship with a woman, not a man. If I found a woman with whom I wanted to spend the rest of my life, I would commit to her in a heartbeat.
My guess (and this is largely based on my own background, each of us sees this differently) is that Willow never really gave it much thought until she met Tara. The chemistry she had with Tara was powerful enough to draw her in and get her hormones flowing in a way that she knew what she needed to do.
And that's another HUGELY important point - it's about people, not labels. For Willow, it was about being with Tara, not about "becoming" a lesbian.
At some point, you might know with absolute certainty that you need to be with a woman or you need to be with a man. Until then, allow yourself to feel whatever you feel for whoever you feel things for. No, I'm not suggesting you toy with other people irresponsibly and go around breaking hearts. You don't have to act on everything you feel. I am suggesting that you allow yourself to experience all that you are feeling instead of trying to bury any of it trying to pretend you're something you're not. That's the only way you can really figure it all out for yourself.
I'm a breast girl myself