5-10%.
This situation sort of happened at a party, although not to the extent of actual intercourse, but to the point where the male involved earned the nickname "Rapist" for the rest of his college career.
The girl was passed out drunk, on the floor, obviously dead to the world. The Rapist (whose previous nickname was "Lower Case" (for 'fail' - there was also a Capital F for Fail who disassociated himself with Lower Case after the rape incident)) Rapist proceeded to dry hump her unconscious body, make out with it, and have his buddy take pictures of him doing it. He was stopped and kicked out of the party before he succeeded in dragging her into a bedroom closet. He was also never invited back to another party again.
Since nearly all of the remaining males at the party were completely horrified and grossed out, except for Rapist and his partner in crime, I'd say a lower number of guys would actually go that far when confronted with the opportunity.
Interestingly enough, Rapist "found Jesus" shortly after leaving college, got married, and has kids. God help the world . . .