The boys were not being molested, they were being young and stupid. It does not sound like this behavior was coerced in any way. There was no adult encouraging it. Going nuts about normal but distasteful adolescent behavior reinforces the unhealthy idea that the human body is something to be deeply ashamed of. This actually makes perverse "fetish" like behavior more attractive to some. It also really does trivialize the how truly wrong real molestation is.
If the embarrasment of his mom catching him in such foolishness is not already a deterrent i would have a brief chat with him about this sort of thing not really being appropriate, or perhaps have an adult male talk to him but I would not make a mountain out of a molehill. I would also do some reading on male sexuality. Bear in mind that the boy is also reaching an age where curiosity about the female form is becoming greater so you may want to be sure and put a decent net nanny on the computer.
try some of these links for info on childhood sexuality.
Child Sexual Development Humboldt-Universität zu Berlin The Kinsey Institute - Home