Standard disclaimer: Real rape, when a person is forced to have sex against their will is always wrong.
What is that fascination of rape?
I think there are a few different things going on that make the fantasy appealing. Of course I'm not qualified to make a scientific, exhaustive list, nor even to prove my hunches. They're just hunches, but here it goes.
The social pressure to be chaste packs a big wallop. Women are mammals, a species that perpetuates the species by sexual reproduction. So the evolutionarily sound assumption is that most women want to have sex to perpetuate the species. So despite the social pressure to believe otherwise women want to have sex.
(Of course people reading this article might have attained a greater degree of liberation from cultural mores, and might not see it this way, but by and large, despite the 60s, despite feminism: Women and girls in the cultures I've lived in still feel that to be sexual, or to invite sexual encounters is immoral --or as it might more likely be expressed-- slutty, skanky, dirty, loose, tease, flirt or whore.)
But even though women are biologically programmed to have sex the overwhelming mandate of society is that only bad women want to have sex. (Which really means all women are bad, which perhaps explains the blame Eve takes in Biblical "scholarship," or the clothing restrictions of many Islamic sects.) Only a woman who is immoral would want to intice a man to have sex. That's the message and since many of us want to fit and and be loved, we fall for it, even though it's illogical and insane.
So how can a woman safely have sex without being immoral? Well if she had sex against her will than she would be innocent of the crime of trying to have sex. But of course people don't want to be raped because this interferes with the desire to survive. So what's a girl to do?
Well it's pretty safe to fantasize about being raped. But you might want to step it up, so maybe you can get your boyfriend o play out the fantasy with you. You know full well the whole time you are in no danger of being killed or seriously harmed --maybe a luxurious after-glow of chaffed and sore, but not maimed. Everyone gets laid! It's a happy ending.
It seems kind of silly that this kind of simple mind game could work, but it seems to be rather effective. What I mean by saying it's silly is: It's obvious since you both talked about it that it isn't rape. And moreover, if the woman brought it up than she is propositioning someone for sex, which violates the whole idea of avoiding the shame of being sexual!
If we extrapolate this thinking out you just might end up joining BDSM web sites and searching for that perfect Dom/Domme. Because now you'll need to be punished for being so sexual! And then we have a big ball of fun!