Americans are weird as fuck over cutting off bits of flesh from your cock. Psychotic behaviour, particularly when they are mutilating children with no say in the matter. Objectively insane.
the joke is that the man does not have boundaries (human toilet? sure whatever - note he's not enthusiastic about it) while the woman does, specifically over something he had no input in.
i guess he could always get an adult circumcision if he really wants to use her as a urinal.
From the different colors of the two speech bubbles, I'm pretty sure HE'S the human toilet, and she's begrudgingly ok with it. Whereas foreskin is apparently a bridge too far.