When you realize something terrible might happen, something you can't protect yourself from, you can't even admit it to yourself.
I guess when your body isn't able to protect yourself, your brain tries to.
Convincing yourself that what is happening is normal. That everyone experiences it. That even if you don't want it, you should accept it and take it anyways.
It's the most humiliating thing that can be done to you when you truly don't want it. It's the most vulnerable you can be. No matter how much you drink, or how little you wear, nobody has a "right" to your body. Your body is yours and yours only.
Go to http://www.ibiblio.org/rcip/internl.html for rape crisis hotlines.