different contexts have different consequences. acting like sending 2 girls 1 cup to HR is the same as sending it to a friend is deliberately obtuse. Acting like mooning someone you know is the same as mooning a room full of toddlers is insane. obviously if you are mooning people who do not want to be mooned then it is harassment. I would not willingly joke with you in a real life environment because I know you've got a stick up your ass. But if I am interacting with a normal person, with normal boundaries, who I've known for a while, then I would feel comfortable mooning them without doubt because normal people do not get upset about it.
If your counterargument is that there are people who do. Yes, there are. And so you don't make jokes without their consent. But I guarantee you on the godfather movie set nobody was upset that they showed their asses.