This is just a hypothesis. Please do not chase after me with knives.

One part of the reason rape fantasy is so common among women is due to sexual repression. They may feel guilty about desiring sex, feel it is improper for a woman to engage in sex outside of marriage, and perhaps look at masturbation as immoral. However, due to hormones, they still have sexual fantasies, which tend to be run by the following syllogism:
1. I'm a good girl.
2. Good girls don't do this willingly.
3. Therefore, I am doing this unwillingly.

I guess that rape fantasy is common among men too, but what man in his right mind will admit to it?!