You're assuming he was sitting upright. You have to put yourself in the situation.....they were playing a sex game with a loaded gun, listening to Shania Twain.....let your imagination go wild here.
You're assuming that the gun went off while the gun was lying perfectly flat on the floor. It seems to me more likely that someone setting a shotgun down (especially someone who is drunk) might not have held the gun perfectly parallel to the floor at all times. More likely is that she was setting it down on an angle, and the barrel was still pointing up when the butt of the rifle touched the ground, causing the discharge.
SD