I knew I was bisexual as soon as I first heard of bisexuality. It was one of those, "Aha!" moments. :-)

Before that, although I knew I was attracted to girls, I thought I was heterosexual since I was also attracted to boys. I didn't really think about it in detail, though -- I'm sure I would have recognized how stupid that was if I'd given it much thought.

Oh, and I had this revelation around the same time I first had sex with a man; I hadn't yet had sex with a woman at the time. I don't really know whether I realized I was bisexual before or after I had sex for the first time.