I think M is like alcohol...having a beer or a glass of wine is not a sin. Getting drunk and banging your neighbor's wife...By the same token, if M leads you into deviant behavior it is a problem. My teacher noticed that my daughter, too young to know what she was doing, would touch herself in class. The teacher redirected her without calling attention to her. We discussed with our daughter that is was OK, it was her body, but that is something she should do in private. I really don't see anyone going to hell over this.

God grant me
The Serenity to accept the things I cannot change,
The Courage to change the things I can,
And the Wisdom to know the difference.