Oh, we're gonna discuss studies now? Studies don't know crap about females sexuality if you want to get into it--when I took psych of women, we discussed at length how there are studies on gay men, but it's very hard to FIND a sample of lesbians. They have to look for lesbian communities really hard, and even then, they're not great samples. And then it's even harder to find samples of bisexual women, because in gay communities and straight communities, bisexual women are sort of in the middle, not accepted fully by either group. People look at them as greedy or lying to themselves.
For many women, and this is backed up by several studies that I read in some psych book called "Selected Readings" on women or some crap, women don't lump themselves into a category (straight, gay, bisexual). Generally speaking, it's a continuum of their attraction to any sex. I'd say culturally speaking (our culture at least), girls place less importance on their sexual orientation and more on finding a partner.