Wow, there've been lots of good posts on this. I definitely agree with
angela146 in that, if you're a guy and can't imagine having a cock in your hand, you're not bi.
(Or gay, but that goes without saying.) I'm also behind
tisonlyi's point that having one or two non-gender-specific fantasies doesn't necessarily make one bi or gay.
I think that what it really comes down to is novelty. For the bi-curious, having sexual relations with a member of their own sex is going to have a feeling of novelty to it. "Look at this fun, new thing I'm doing!" The true test then, I believe, is their feelings when it stops being novel and new. Is it still fun, or is it something you've done and didn't mind, but don't care about?
(Of course, for some, it might not ever lose its novelty. . . . )