Poundtown's Law of Reciprocity
A non-written but universally understood law stating that what a man does for a woman during coitus, she must do for him in return.
Bro 1: Dude, I hooked up with this skankbag I met on Tinder. I went down on her and she refused to fellatiate me back.

Bro 2: Did you explain to her Poundtown's Law of Reciprocity?