As a law student, I've got to say that rape is not as cut and dry a concept in legal terms as it is on those posters you find in college dorms. Well-- in some states it is. in others, it isn't. Rape laws vary from state to state. Some states require force-- without force its just "sexual assault"-- but ideas on what rape is often include lack of ability to consent, simply saying no, or the lack of an affirmative acceptance.
So grey rape are the situations where a girl gets drunk and participates in the sex, or where she's sober and says no, no, no, but then says its ok, when there's coercion or pressure, when there's anything questionable really.
There is one specific scenario of grey rape that I found worth thinking about more than others-- the mutually drunk situation. Let's look at two scenarios, each involving one night stands.
Scenario 1: A girl and a guy are both drunk. They head back to his place. There is mutual fooling around, and then mutual sex. So, both partners are in the affirmative on this one-- but they are both drunk. Both think they know what they are doing, and both are as aware as drunk folk can be.
Scenario 2: A girl and a guy are both drunk, again at his place. There is mutual fooling around. All of a sudden, the girl gets flipped over, and he's in her. She isn't participating anymore, she's laying there limp-- but she also isn't stopping him, and she didn't say no. She didn't even realize it was going to sex until he was in her, because she was that wasted. But, she was enthusiastic on all the stuff before sex, so how is he to know to draw the line there?
Moving outside of legal terms, and into more moral/philosophical ones, I personally don't think either situation is rape-- largely because BOTH parties are drunk. So technically, if she can't consent? Neither can he. So who is to say one of them is raping the other. Even if, in both situations, she wouldn't have consented if she were sober-- who is to say that he would have consented if he were sober? What if neither the guy nor the girl in either situation would have consented when sober? What if its just the details of the situation-- say, the guy in situation x had a girlfriend, and the girl didn't approve of cheating, but would have had sex sober if he'd been single, but he still wouldn't have?
Consent is not usually tricky. You get an affirmative response, you don't sleep with drunk people. Easy. But when both individuals are drunk? It gets confusing, to me, at least. Is anyone actually raping anyone else here? Its tricky. And I'm not entirely resolved on how I feel about it. But at this point? If both individuals are drunken idiots, and no one actually says no OR gives any physical sign of no? Then I don't think it is.