The Cleveland-Loretta Quagmire - S4-E5
Plot hole: Brian and Peter see Loretta and Quagmire having sex yet, they aren't sure who she's having an affair with. There's no way they couldn't have noticed it was Quagmire - the couch was facing the front door! And obviously Quagmire was on top of Loretta considering the fact that he leans up, says something, then she tells him to get back down here and finish.