No, it is not a sin. Even if it were a sin, it is not nearly as bad as other things that are expressly listed as sins. What about gluttony? That one is way worse, yet I am sure there are scads of gluttons here- myself included. As far as I can tell, masturbation hurts NO ONE, while gluttony is detrimental to health, and therefore families are affected as they have to take care of people who prematurely ruined their health.