Just saying he might be stupid and move the wrong way, or the one on top might lift too far up. It hasn't happened to me, but sex has it's issues, same as walking on the streets and other random shit requiring two subjects working in opposition.The "Cowgirl" position is one of my favorites. I enjoy being on top and in control. The key word there control. There is no need to go crazy and fracture his penis to achieve the pleasure.