I don't know that the most common fantasy is specifically rape. Instead, I think many women fantasize about being seduced and willfully overpowered. Naturally, we are built to be very aesthetically pleasing to men, and I think we are instinctively turned on by feeling like we are really turning a guy on to the point that he 'has to have us', when it's happening in a fantasy world where there aren't any societal restrictions about going along with it.
Thus, the fantasy of being dominated is a turn on because we are actually dominating the man's attraction to us, causing him to need us and to want to overpower us. By being dominated, we are dominating the man because he has to have us, wants to use us and do what he wants with us.
However, I am taking care in the wording of this; the fantasy of being dominated is probably NOT often paired with a desire to be physically hurt, beaten, or seriously afraid. And no, we do NOT want to be raped in reality.