Why would I fantasize about gay sex if im straight?

Fantasies are just that, fantasies! Rarely does one act upon a fantasy, and I know many straight men who have fantasized about having sex with another man. They certainly fantasize about women have sex with each other, but rarely will they vocalize their fantasies about men. I, as a gay man, have on occasion, fantasized what it would be like to have sex with a woman, doesn't mean I will act upon it. Sexuality can be very fluid...