Thanks for that, Evan. It's not an uncomfortable question at all. As they say, whatever consenting adults want to do is fine. The question is, can she consent to this?
There's a concept in criminal law that one cannot consent to being assaulted, because assault by definition is unwanted. Thus, if two guys get into a dustup at a bar but nobody is hurt, charges are unlikely because they both entered into this understanding that they'd tussle but not seriously hurt each other. If a weapon comes into the tussle or one person injures the other, or keeps going after the other when the other has surrendered or is unconscious or incapacitated or clearly drunk, then charges will usually follow. That makes sense when you think about it.
The question is, can someone consent to be raped? By the same analysis, if it's on consent then it isn't rape by definition. But you're right to be concerned because the police and Crowns hear that defense in many rapes, that "she wanted me to" or that it was consenual and got a little out of hand. And that defense is on the table because in most cases of serious sexual assault, the parties know each other if not actually in (or were) in a relationship.
If I were a therapist, and I'm not, I'd dig a little deeper into why she wants to live out this fantasy. It may well be that she's had a conservative and sheltered life and now she thinks that doing this is going to be kinky, fun, really "coming out of her shell". Or maybe she recently saw some violent pornography and she found it stimulating. Or maybe a friend told her that this kind of sex play is really hot, and she's looking for the same thing without knowing what she's getting into. Another possibility is that she's testing you to see if you have any violent or misogynistic fantasies of your own.
Clearly, you need to be careful about how to proceed and assume that she doesn't really understand what she' asking you to do. For the fantasy to be realized, she is saying that it needs to be "real", with no warning or notice or anything. That crosses the line into consenting to being violently assaulted which one cannot do by definition.
One obvious solution is to get her to sign something in which she acknowledges that she's wanting you to do this, but even then it's dubious as evidence.
If I were you I'd approach it this way: start off with something less severe. I'm not suggesting whips and handcuffs and choking and leather, but rather some roleplaying, or blindfolding, or simple restraints to see how you both react to it. If you're uncomfortable with what she's suggesting right away, then she should respect that too. The most important thing is to have a "safe word" which when spoken means that the activity instantly stops and the game is over.
There is absolutely nothing wrong with exploring your sexuality and trying new things in order to satisfy each other and learn more more about what satisfies you. Just be safe, be careful, and make sure that you both know what you're getting into. And that's the obvious concern here: she wants to go directly to the extreme right away.
There's also the risk that she'll be traumatized and call the police, or more likely tell someone about it who will call the police. And whether you're convicted of anything or not, nobody wants that kind of arrest in their history. While it won't come up in a standard records check, it's still there and can come up. It could be noticed if you're trying to cross into the US (border personnel databases show nearly everything), or in family court if your complete records are brought in under subpoena.
Does that make sense? Please reply if there's more to discuss. If I've answered you fully, I'd appreciate a positive service rating please.