If you're having relationships with women, or are about too, the fat of the matter is that your bisexuality is something actual and tangible to you. You don't need fantasty to "back that up."
Or even relationships, really. If you have tangible attraction and interest and you know what you know, that's all you need.
It's common, during developmental phases of our life, for sexual identity to be far more fluid than other times. So, the one thing possibly to think about if you're still uncertain is how you might feel about coming out, then potentially a few months or years from now, finding your feelings have changed, and having to "come out" again as a hetero or homosexual, whichever way you may go. That may or may not happen, but it does for plenty of folks, some of whom find that having to essentially say "Well, I was wrong" or "Actually, I'm this," is very uncomfortable for them.
So, it never hurts to think this stuff through, think about what your agenda is in coming out, etc. But given sexual identity often IS very fluid, it is okay for it to be such and to be out in every developing state, as feels best to you.