Thanks, Kitchenpete. You saved me from an obvious error. I was going to differentiate sex and masturbation as the difference between a social and a solo event. Woody Allen shows that both are social. In masturbation "I" have sex with "me."
BUT ON SECOND THOUGHT: when you do it, "you" are having sex with "you."
In other words, when I do it it's sex (social). When YOU do it it's a masturbation (solo) .