One University study suggests that primates have been masturbating for around 40 million years. I think it’s safe to say we’re probably ok beating our meat a little now and then (though obviously excessive masturbation can be an issue). Interestingly, prevalence of masturbation seems to go up with primates in captivity, which to me suggests it’s either a stress response (relieving tension), a reaction to the more limited activities available (boredom), and/or a reaction to the safer environment (more free time not worrying about getting killed).