I can see two answers to the apparent contradiction.
The first is sexual identity. I put off replying for this long because I'm not terribly well-versed on the subject. My roommate is, though, so I asked her. Here's my attempt at transcription (couldn't quite keep up) of the relevant part of the conversation:
Her:
I would argue that your sexual identity doesn't have as much to do with what you've done as it does with how you feel about what you've done.
I'm a lesbian - that's how I identify because I've had sexual experiences with men and I didn't like them. I didn't like them and it wasn't pleasurable at all.
Me:
What drove you to it?
Her:
Drove me to... sexual experiences with men?
Me:
Yeah. Just... why?
Her:
Being attracted to them, I guess. But over time I became less attracted to them. Dudes can still look attractive to me, but I'm not going to do anything with them.
By the same token, if you're a straight girl in college, and you think some girl is hot and you try something with her, if you like it and go on to continue doing it, yeah, you're bisexual. If it wasn't really that great, and you don't do it again, you're just straight.
Me:
Can you see conceive of yourself screwing a dude? Like... everything's perfect, whatever it is you're looking for, whatever you imagine.
Her:
Hm. Yeah. But it's not gonna actually happen.
My angle is the usefulness of words. A bisexual person feels sexual attraction to people of the same and different sex. Not feels sexual attraction to the same sex and had a sexual relationship with a person of different sex for a year twenty years ago. If every "straight" girl that ever felt sexual attraction to another girl at any time in her life is secretly bisexual, the word bisexual loses its usefulness. Why use the word bisexual to describe someone that's only ever going to want to screw one sex for the rest of their life? What should we call the people that experience significant sexual attraction to both sexes? Isn't it more useful to call me bisexual and my roommate homosexual than to call us both bisexual?
Here's the tired old analogy again. Suppose you're Japanese. Your parents, grandparents, great-grandparents, keep going back, they're all Japanese as far back as there were humans on those islands. Except one greatn-grandmother that was... Finnish. What is the value in describing yourself as Finno-Japanese instead of Japanese?
I don't care to keep arguing this, and probably shouldn't've bothered with this post. I hope it's helped your perspective some, though.