Speaking as a guy, all I can say is, ouch. As a doctor, however, I can tell you that this injury medically known as a penile fracture, is rare. Still, I've seen it in the ER, and it isn't pretty. It's a true urologic emergency.

How It Happens: During girl-on-top sex, a woman will pull out all the way, then thrust back down really hard - inadvertently slamming her pubic bone against her guy's erect penis. And in a battle between his penis and her pubic bone, the penis loses. One of the blood-filled tubes of tissue inside the penis can rupture, causing a snapping sound, agonising pain, and purplish swelling and bruising. Did I mention the agonising pain? If a guy gets to the hospital pronto, surgeons should be able to repair the ruptured tube and hopefully, in a few weeks, he'll be as good as new. If he waits, his penis may not heal properly and he could end up with a permanent curve and/or erectile dysfunction.

To keep your guy from this horrific fate, try not to thrust so high or so wildly in the girl-on-top position that you risk losing control of where your pelvis will come back down. And if he's the one thrusting out of control, tell him to slow down… or potentially pay the price.