The thing that has failed to be mentioned is that rape is more about power than it is about sex. Those fantisizing about the raping, I believe, really crave power. On the other hand, those fantisizing about being raped want someone to have power over them. Someone having power over them thus shifts the responsibility of the act commited to the aggressor.

So, perhaps it is a passion that is not socially acceptable... my expansion on that theory is that if someone is forcing them to do it, it wouldn't be their fault.

I am no psychiatrist, but I have taken some classes and unfortunately was also the product of someone taking these fantasies a little too seriously. Rape is a violent thing, and fantasies about it are more than sexual, they are about power... and I don't think they are dangerous as long as they are NOT acted out and the people fantasizing understand why they are thinking that way.