What's a kick in the nuts between friends? We were just horsing around.

But the potential for the illusion of transparency is ridiculous, and such an action smoothly transitions into hazing and/or bullying. Which are also widely tolerated by our social norms, unfortunately, as long as the person being bullied is low-status.

It seems obvious to me that if you slap someone on the back (literally or metaphorically) and they complain, then you should immediately drop all considerations of "insulting people is fun" and stop doing whatever it is you're doing. Even (especially?) if you think they're being a baby.