All of us have probably had them at some point. Sexual fantasies that stray out of our comfort zone, into the realms of something that can be frightening, disturbing, or unsettling, and that leaves you feeling decidedly uncomfortable afterwards. But there's something compelling about these fantasies that draws us to them in the first place. They're the sexual equivalent of peeking through a gap in your fingers, or hiding behind the couch as you watch a horror movie.
As human beings we have an innate curiosity to explore our emotional extremes, and, while things like the cathartic appeal of horror movies and Stephen King books are relatively socially acceptable these days, the idea of dark sexual fantasies is most certainly still a taboo beyond the taboo.
The lack of social acceptance is probably a part of why we're drawn to these ideas and images in the first place (making anything forbidden is a surefire way to make it feel hotter and naughtier), but it's not their core appeal. BDSM and Master/Slave relationships are getting a lot of spotlight in the wake of 50 Shades, and they certainly touch on the disturbing side of an erotic dark fantasy. There's something very worrying about the idea of giving control of your life over to someone else. In the healthiest of relationships this ideally comes about as the result of a strong bond of trust between two partners, but it's almost more exciting if there isn't that bond of trust. After all, the whole premise of such a relationship is a game of make-believe where one person is forced to do things against their will. It ties in strongly with noncon/rape fantasies -- it's all about the emotional unburdening of having control completely taken away from you.
Master/Slave fantasies are one level below this, though. They have the safety net of knowing it's all still a game. A good Master doesn't really want to abuse and humiliate you; he's doing it all for your own pleasure as much as his.
But then you move up to the really dark fantasies. We're on the level of nonconsensual sex here (though some peoples' fetishes extend even further beyond that -- and that's a place even I'm not comfortable delving into!). For the longest time I didn't get this kink. It was very disturbing to me, and I couldn't see how anyone could derive pleasure from the idea of being forced to have sex against their will.
I think the key to understanding this type of dark fantasy is to underline the fact that it is a fantasy. The real life equivalent of this is harrowing, horrific, and absolutely not desirable in any way. For a lot of people (myself included, for many years), the distinction between the two is very difficult to make, because it's such an emotionally charged subject.
The best example I could give would be to equate it to fictional violence. I'm not someone who enjoys a lot of blood and guts in my movies, but I understand that many other people do. Whole subgenres have sprung up around it in years past, and it's become so socially acceptable that "torture porn" movies have even become a thing. I personally find these pretty distasteful, but, looking at them with an objective eye, I have to appreciate the reasoning behind why they exist.
As I mentioned at the beginning, we're creatures of emotion, and fiction gives us a safe space within which we can explore the most disturbing extremes of the human condition without having to deal with the consequences. Through dark fantasies we're able to explore the primal parts of ourselves that still crave something we've come to understand is wrong.
I can't see myself ever writing something as dark as a full-on rape fantasy, but I've certainly brushed up against the idea of dubious consent a few times (something that'll be recurring in my Wild Instincts serial). The wonderful thing about fiction and fantasy is that it allows us to do and experience things we wouldn't otherwise dream of. For anyone who enjoys taking the occasional dip into their darker side, the line between fantasy and reality is very important.
Just because you dream of being forced and taken by an unstoppable alpha male, it doesn't mean you want that to happen.