Proof? It's known that when you masturbate, you ejaculate your reserve and your body creates a fresh batch. The newer the sperm, the healthier it is, no matter how it gets out of you.
"Australian researchers have found that having daily sex actually improves the quality, if not the quantity of sperm. The thought behind this is that regular ejaculations get rid of the old sperm and make way for newer healthier sperm."
Daily sex can be replaced with daily masturbation, as your body doesn't know the difference.