Does masturbation causes muscle loss?

No: Because muscles are being used (for example, the muscles in the hands and arms), the muscles can become a bit more toned but probably not larger (unless one is spending way too much time with this activity).

No: Somebody has told you a lie to try to frighten you. This is somebody you probably want to avoid. You are a 19 year old man and essentially all of your peers enjoy their bodies in this way. It helps keep you focused and able to do other things you need to do; it also lets your learn about your mind and body to prepare you for adult life.