I don't know what fraction of women attending college are victims of rape there now -- for reasons already aired by others in this thread, the figures often quoted don't seem terribly trustworthy. But I doubt that an official no-sex-before-marriage norm, as such, makes much difference.
Explicit social norms of this kind notoriously make less difference than you might think. (Warning: most of the things below are single studies which could be wrong or misleading in all kinds of ways; but I know of no reason to expect them all to be wrong in the same direction.)
But I bet a no-sex-until-marriage norm was quite effective in making it harder for women to say they'd been raped. Not for the stranger-leaping-out-of-a-bush sort of rape. But for the sort that I think is much more common on college campuses, where someone gets you too drunk to resist or slips drugs into your drink or just declines to stop when you say "look, this feels very nice but I really don't want to go any further" -- well, with those social norms in place you can't tell that sort of story without getting labelled a Bad Girl, in which case you lose even if your story is believed.
I've started a podcast called Future Strategist which will focus on decision making and futurism. I have created seven shows so far: interviews of computer scientist Roman Yampolskiy, LW contributor Gleb Tsipursky, and artist/free speech activist Rachel Haywire, and monologues on game theory and Greek Mythology, the Prisoners' Dilemma, the sunk cost fallacy, and the Map and Territory.
If you enjoy the show and use iTunes I would be grateful if you left a positive review at iTunes. I would also be grateful for any feedback you might have including suggestions for future shows. I'm not used to interviewing people and I know that I need to work on being more articulate in my interviews.