Frankly, I don't support the idea of being able to withdraw consent after the fact, at all. He didn't call you back like he said he was going to? Well that's sad, but it's not rape. You were totally into it, but then he came and got up before you had yours? Shitty deal. Not rape.
In other words, you should know when you're being raped, while it's happening. It's not that hard to figure out "hey, something's happening to me right now, and I don't like it. I want this to stop." If you let it go and then you feel bad afterwards because you wish you would have stopped it, that's on you.
It's a problem of education, teaching women (and men for that matter) that they own their bodies and they control what happens to them, at every moment. You never "have to" give up the goods, no matter how many Margaritas he's bought you, no matter how far you've let it go before you change your mind. Teach women to have self-esteem and the date rape problem pretty much takes care of itself.
I think there are also cases where one person is very aggressive and enthusiastic and the other is shocked and so doesn't say no right away, and then has this feeling that it's "too late" to say no
I was thinking about it this morning. Seems to me that even if the aggressor did take a second to ask "do you mind if I continue?" that someone who's so shocked and already feels it's too late to say no, is likely going to say "go ahead." If anything, the simple act of asking might make the person appear less aggressive and therefore even more likely to get explicit consent. So where does that leave us?