Hmm, don't know how I ended up in this thread. I've always wanted to read Mark's stuff. Just too lazy to have gotten around to it. Maybe I'll start....
Anyway, I'll answer from the perspective of a 19 year old guy who identifies as bisexual, leaning male. I've never had sex with a girl. My entire realm of sexual experiences are confined to a couple of guys. I've done everything from bottoming to topping to sucking to getting sucked and most things in between. In high school, when I think back, I had several serious chances of getting laid with girls. I found them attractive and wanted to, but a part of me kept telling myself that I was gay and thats not what gay guys do. To me, you were either gay or straight with no middle ground.
Now I've matured to realize that there is a lot of middle ground, and while I fall closer to the 'prefer males' end of the spectrum, I don't putting having sex with a girl out of the possible future. Its not something I'll actively seek out like I do with guys, but if the chance comes and I want to, I won't say no.