I appreciate that you are sensitive to this issue! Here’s the thing… rape is non-consensual. That means unwanted sexual encounters. Lots of people have “rape fantasies,” but they aren’t so much about the actual rape, which is undesirable, as they are about the idea of someone being so consumed with lust that they have to overpower you to have you. It’s all about power, and as such is quite a common desire in a D/s relationship. Your girlfriend would very likely find herself terrified of an actual rape encounter (see also: The Piano Teacher), be it with a stranger or someone she knows (most victims know their rapist).
What she is expressing to you, instead, is that she wants you to exert power over her because it turns her on—she’s just using the wrong language. My best advice is to talk to her about what kind of things she would be okay with (ex: choking, slapping, being tied down, having clothes torn) and always, ALWAYS make sure you both have a safe word or action. Stop if either one of you wants to stop. You don’t have to feel guilty for doing things she asks you to do, as long as she is capable of giving informed consent (i.e. not under the influence or underage) and you always have her safety and best interests in mind.
Here are some links that might be helpful to you: