Morin makes an observation that I find very credible and a key piece of the answer to your question. Both top and bottom feel strong and affirmed. Ultimately, control resides within neither alone, because the energy is generated by their interaction. Anything from cartoons, to movies, to novels. My hunch is that the first time or two is coincidence, but then the pleasure of sexual arousal helps the person cope with discontent or anxiety surrounding his or her powerlessness.
Pleasure makes the frustrations more tolerable.
What seems to be true from Morin's research and my experiences with clients is that, for people attracted to these types of fantasies, a strong pairing occurred between images of power and feelings of sexual arousal. That's the first step. The second step involves the issue of one's own desirability. If a person wants to be desired very strongly, being forced may have that implication. And here's the third step.