When I was that age this type of behavior would of been considered homosexual in nature. Nobody would do this unless they were a pervert. When I read some of the comments from people who see this as nothing, I wonder about them.
And the flip side of this is that if the boy put his naked butt on a girl’s face, no one would be laughing it off.
I agree this is not horseplay. Its obscene and unacceptable. For a kid to think this is fine, as an adult would be doing worse things...