I'm with psychehedone. I think for many it really is a submission/rough sex fantasy and that these particular fantasies have been mislabeled.

Having said that though I have seen some people who genuinely believe they want to be gang raped in a dark alley by strangers, perhaps this is really what they want.

But having been assaulted myself I tend to think these people really have no idea of what they are asking for. They're thinking entirely of the sexual part not of the emotional trauma of being made completely and utterly helpless.