I've known several "bi" women (including a girlfriend in college).
Well, I like the Dennis Prager theory:
Women make emotional attachments that sex goes along with, and they can have them with women or men. In a normal life, they form them with men.
Men are just horny, and some horndogs will find recipients who are male, female, animal, vegetable mineral, whatever. In a normal life, they are horny with women.