For what it's worth....I work in child psychiatry and we see a lot of "bisexual" teenage girls and the occassional boy. It's become a label, almost, like being a "headbanger" was when I was young. They are no longer "emo", they are "bisexual". I'm not saying some of them aren't really bisexual or experiment with it a little bit but most of them really have no idea what it means it's more of a statement for them. It's become almost common place and a lot of them talk about having gf's and bf's at the same time but when it comes right down to it, they aren't out sleeping with everyone. A generalization but you know what I mean.
That being said, I'm not sure how I'd feel about it either with my teenage son. I'd like to think I've seen just about everything and yet, when it follows you home, it's different. I've gone the route that hitman talked about...education, education, education.