I saw an interview with Asia Carrera once where she said she only likes to do DP on film, because when she tries to do it in her personal life, the guys don't want to touch each other. They don't seem to mind if they're doing it on film (since they're paid to do it, I guess).
But this subject was the funniest part of Harold and Kumar. When the girl tells them she wants to do them both at the same time, and Kumar's like "Yeah let's do it"! But Harold hesitates and Kumar's like "Why not?" Then Harold says "I don't want our balls rubbing against each other!" LOL!!!