Parenting Teenagers (12-18) Support Group
This community is focused on the joys, challenges and concerns faced by parents of teenagers (12 to 18 year olds). The major areas of child development include: physical development, perception and sensory development, communication and language development, cognitive development, emotional development and social development.

1) it is a n'normal' boy activity so do not panic
2) it is not a normal activity for a parent to endorse, so do react
That's my opinion in a nutshell.
Finally - it is best for you to address because it is your feelings; asking another parent to accept a separate point of view is different from asking them to carry the banner.
My rule for my son is to not be a pervert about it. I believe it has all but stopped over the last 3 years.