I'm pretty sure most people who make rape jokes don't condone rape; they're just trying to be funny. Humor is rooted in pain; it's how we cope with things and, as McJob said, "clear the air" about certain subjects. Slapstick is funny because we can all sympathize and empathize with the person. Rape jokes are funny (to some, to others they aren't and that's fine since humor is subjective) because we can empathize and sympathize about how horrible it is.