@eobox91103:
Another way to ask the question is, "Are there people who are sexually attracted to both men and women?" The answer to that is demonstrably yes–it's simply a fact of human nature. It's a separate question whether to label this as bisexual, pansexual, ominisexual. As Anything notes, this second question has to do with social systems, not people. I tend to dislike labels.
I'd have to think about your new question, because it sort of indirectly open the doors to that whole "fluidity" concept. I emphatically reject the notion that everyone is sexually fluid. I can remember being attracted to dick before I even understood what it was. I had absolutely no desire to try out pussy!! Fucking yuk. Now as an adult, the whole subject of transgender scares me. It scares me in a way that I wonder if it is what straights feel when they discover a co-worker is gay.
-AOS