One should also not shy away from the dark side of masturbation. The typical associated lust has been mentioned a couple of times. Lustful thoughts are sinful and your teen needs to know that. Masturbation can also be addictive and very frequent masturbation, especially in girls, can cause physical damage. Masturbation can affect later marital sexual relations both in terms of desire and in terms of sexual enjoyment. There are documented cases of both men and women who developed such personalized masturbative practices that they could not have orgasms any other way. For males, the increased pressure associated with self-stimulation vs. the pressure experienced in intercourse can be off-putting and make sexual enjoyment from intercourse less, well, enjoyable.

All in all, if you agree masturbation isn't itself inherently sinful, it still should be rarely practiced for the best sexual health and development. If your teen is old enough to do it and mature enough to talk about it, they also should be open to hearing about the many physical, mental, and relational dangers of the practice.