I think it's a complex issue. I mostly agree with SC, but 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 think people need to be told that it's never too late to say no, and from both sides. That is, "say no whenever you're uncomfortable" but also "stop whenever someone says no or otherwise is showing that they don't want it to keep going"(like pushing you away or facial expressions).
I don't think it's fair to call someone a rapist if, from their point of view, their partner was enthusiastically partaking. But just laying there doesn't imply consent either. Because someone didn't have the confidence to assert themselves doesn't mean it's "their fault" if the other party just went along and never bothered to look at the signs that it wasn't consensual.
I think permission before every kiss and time having sex isn't really something I require. But if someone kissed me out of the blue I would definitely be pissed off. And depending on the circumstances, I can imagine someone starting to undress me causing me to freeze and panic. Saying "no" means trusting that the person will stop if you do, rather than beating you up, and it might be hard to trust such a thing from someone who just started fondling/undressing you out of the blue.
So I think there is a range. I definitely agree that a discussion beforehand, like SC suggests, is a good thing. However, saying that if you're not able to have that discussion, you're not mature enough to have sex does nothing to help with the fact that some people don't have the discussion yet still have sex. Nothing is stopping them from that.
Maybe people should be taught about that first discussion early on so that they're more likely to have it. I have to say that if having such a talk "breaks the magic" or whatever, chances are having sex with that person was a bad idea in the first place, so no harm done. Sure, some dates might go better if people act without talking, but that's only because they don't get a chance to realise they're not compatible. You don't want a date to go well only because you don't know better. The later you realise you were not compatible, the more time you've both wasted.