Most of the time, they don't get to choose who they have sex with, which means they have to get it up for any girl. There's tons of lights, people watching and it's a lot of pressure. You have to cum on spot when they tell you, and you can't get off too soon either. You have camera's coming at you at every odd angle, and the sexual positions aren't very comfortable or realistic.
Not all men are used to/can handle that amount of pressure.
http://media.ebaumsworld.com/mediaFi...01/1033850.jpg
Reminds of this picture. NSFW.