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.)

• Divorce rates are higher, not lower among evangelical Christians.
• Of all US states, Utah views the most internet pornography per capita.
• Abstinence-only sex education correlates with higher teenage pregnancy rates.

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.