Besides, if you are concerned that they are showing each other their bodies you have more to worry about than snapchat because it is going to happen regardless. At least nobody has got an STD from snapchat.
And after the things I found when doing a backup of my 16 year old's computer I'm more concerned with the pictures that stick around than don't. We had a talk about why I never want pictures of naked teenage friends on my network.
But if you think you can stifle teenagers, you are wrong. You just push them into the shadows and you out of their life.