Not the first (maybe the first sitcom.) There was a great scene in Thirty Something: Billy (Ellen's fiance) is tired of listening to her psychotic babble, so he walks away in mid conversation, into the bathroom, drops his drawers, and sits on the can. The camera is in the bathroom, showing Billy sitting on the toilet (waist up) and Ellen standing in the bathroom doorway. When she fails to take the hint, he says something like "excuse me a minute" and slams the door in her face. Not the type of obvious humor of a sitcom. It's funny not because they're showing some guy actually sitting on the toilet, but because many couples can relate to Billy's tactics for avoiding the conversation!
Robert