"Don't touch my butt"

Why do I even have to say this?  Shouldn't it be a duh?
Often when I'm just standing, one of my kiddos will just touch my butt. I don't know if its at the right height that its irresistible,  or if its just a phase. But I don't know how to handle this situation.
I had to say to him, "Don't touch my butt, would you like it if I touched your penis?!" Thank God he said "no."
Please just be a phase.