In women who are more sexually repressed or who have feelings of shame surrounding their sexuality, fantasizing about rape could be a way to be released from the responsibility of their actions, and to be able to feel pleasure without shame.
I do hope that we will one day live in a world where sexuality is no longer a source of shame. In the meantime, rape fantasies can provide an outlet for women (and men) who want to be free to experience sexual desire and pleasure without all the negative feelings it usually provokes in them.
The Desirability Explanation
This explanation is less common, yet more compelling for a wider range of women who have rape fantasies. The dynamic is simple: I am so desirable that they can't help themselves.
Research has shown that unlike men's desires, which are raised simply by seeing someone desirable to them, women react to feeling desired. The more desired they feel, the more likely they are to want the person who has desire towards them.
Rape fantasies give women an opportunity to imagine feeling so desired that the other person literally cannot stop themselves. The intensity of this desire is what women feed off of for their own sexual arousal. The rape fantasy becomes a way to experience more intense pleasure.
Women have also been conditioned by decades of romance novels to see this kind of "ravishment" as erotic. The heroine, unwilling at first, is swayed by the hero's unwavering, obsessive desire for her, and lets herself be taken - in the hope that the man will now be softened, and ready for marriage (of course!).
How amazing does it feel to provoke such desire in someone that they are compelled to have you? I know that my own rape fantasies have a lot to do with this explanation - and sometimes I go out of my way to make my lovers feel desire for me at random moments through sexting or naughty emails. It is a powerful act for me, something that I can actually control and provoke on my own terms.
The Sexual Freedom Explanation
This is also a compelling and common explanation for rape fantasies: I am free to fantasize about whatever I like, even taboo things.
For sexually adventurous women, fantasizing about rape and forced sex can be a part of exploring the wide range of desires that our imagination can conjure up. It's not so much about the fantasy itself; rather, it's about the person fantasizing. An interesting opposite to the shame explanation, a woman who fantasizes about rape can be totally uninhibited and shameless when it comes to sex.
It's very empowering to be able to fantasize about pretty much anything, even the most taboo desires. Since it all happens in our heads, we can change the scenario when it stops working for us, perhaps by turning the tables and give our ravisher a taste of their own medicine.
Surrender
In common rape fantasies, control is not so much taken away by force; it is surrendered. In this sense, rape fantasies have a lot in common with submissive desires, where a person willingly gives up control.
Rape fantasies involving physical violence may indicate masochistic desires as well. Pain is a turn-on for masochists, and introducing pain and fear in fantasies through a rape scenario can be very satisfying.
So, if you're still looking for an answer: no, your rape fantasies are not crazy. In fact, they are more normal than you think.