Question: If I fantasize about men and women, does this mean I'm bisexual?

Answer: Maybe, maybe not. Many people fantasize about many things that they do not act on. If you are a woman and you primarily have relationships with men, but think about what it would be like with another woman, you may be bi-curious.
How one defines their sexuality is a very personal thing. Fantasy is just that, fantasy. Even if you decide to act on your fantasies, how you define yourself is up to you.