The idea that masturbation is an addiction is a product of religious morality, not science. When you look at models of addiction, they’re compulsive behaviours that are no longer pleasurable themselves, but are done to ward off unpleasant withdrawals. E.g., when you first start smoking, the act of smoking is, itself, enjoyable. But very quickly it becomes something you’re doing to stop the unpleasant feelings of nicotine withdrawal. That’s not what sex “addiction” is at all.