For under-reporting, look here. Even amongst high-school students, the incidence rate was as high as one in five women, and half of these had never told anyone about the incident.

The point here is that feminists tend to use a definition of "rape" that is vastly more general then what the word commonly refers (it tends to boil down to "any sex you regret in the morning") to in order to inflate the statistics.

I'm sorry, but this is absolute nonsense. In fact this is precisely the kind of nonsense that gets used to systematically belittle and trivialize rape victims, and which leads to the under-reporting I mentioned.

The typical popular model of sexuality goes something like this. The woman has, i.e. possesses, sex; the man wants to get it from her. She, on the other hand, wants to hold onto it for the best mate she can find (in order to get married, etc.). Therefore his job is to put on the moves, and her job is to put on the brakes. However, if she resists, then she's a bitch, because he deserves it after all, therefore she better not resist. If she does resist she might just be playing hard to get, because after all she really wants it, so as long as she's not resisting too hard you can keep pushing anyway, either ignoring her protests or whining until she gives in. If she regrets it in the morning, well, she shouldn't have been such a slut anyway. Because this is after all the sexual norm, she probably won't even think of it as rape, and might never think to mention it to anyone.

Feminism makes the radical suggestion that this model is totally, balls-out insane and that maybe our notion of a healthy sexual interaction should necessarily include enthusiastic consent on both sides. If you want a more complete summary of the feminist position, "Yes Means Yes" is a good introductory source. I don't think I can do as good a job of explaining as the authors can, so I'm going to leave this off here.