What am I?

Today's constant thought in my mind; my sexuality.
I've never really talked about it because I am a girl who likes boys. Simple and normal. But lately I have been looking at girls too. I could never really see myself being in a relationship with a girl before but I've always had... I don't know if you would call it a fantacy? But I've always wanted to know what it was like being intimate with a girl. I'm not sure if that means I'm bisexual or what. It would be fine if I was. I completely support people who are gay, lesbian, bisexual, transgender, drag queens... You name it, i love 'em(: Haha. I just don't really know what I'm feeling. Am I just convincing myself of this stuff?
When I was about 11 I started looking at porn but I would rarely be turned on by just a guy and a girl. I thought I was so weird because I was only aroused by watching two girls. (or three, whatever. :p ) But I was listening to 'Love Line' one night and this girl asked the same thing and Dr. Drew said that it didn't mean anything it was just a preferance of porn. It doesn't mean you're gay. I was relieved and went back to my boy crazy self. But now... I'm not so sure. How do you even know you're Bi? And more importantly, how do I know if I'm Bi?