In alt.tv.simpsons, on the "Re: really wimpy line" thread, Jesus, The Destroyer wrote:
I'd say Moe's self-censorship was the joke. Everyone (including the other bar patrons) knows the official phrase, and nobody would have objected to "pissing contest," but Moe cleaned it up anyway. It's the incongruity that's supposed to be funny, but apparently wasn't for the original poster.