Can masturbation cause infertility in men?

No: I'm sorry somebody told you this. It's a claim made by people who want to frighten you for their own dark purposes. If a man ejaculates frequently, his sperm count will be lower because there's less absorption of fluid between events. Enjoying your body keeps you comfortable, lets you focus on other things, and teaches you about your mind & body in preparation for adult life. Think for yourself.