Its pseudo science made up by prudes and sexual guilt trippers to try and trick the more scientifically minded that masturbation and sex is a bad thing. Science actually supports the idea that sex and masturbation is healthy, it actually helps prevent prostate problems in men and has lots of psychological benefits. Ask yourself this, why is it that 99% of these people say that masturbation is bad but sex isn't, physiologically and psychologically, the same things happen.
Of course, there is such a thing as too much of anything, so if you're doing so much that its taking over your life, thats not a good thing.