Even though I've never had sex with a woman, I've been attracted to women (as well as men) all my life, so in my view I'm definitely bisexual and not just bi-curious.
Right now the only thing standing between me and girl-on-girl sex is the unfortunate fact that I haven't met the right girl.
That said, I don't think I could be in an exclusive relationship with a woman (well, I couldn't be in an exclusive relationship with anyone actually, but especially a woman). I just love cock way, way too much. That's why I call myself 80% straight and only 20% gay.
Actually, based on who I'm attracted to it would probably be more like 60/40...it's just that when it comes down to it I don't think a steady diet of only pussy could keep me satisfied, so for practical purposes I'm a lot more straight than gay.