Season 5: Episode 90: Bill and Peter's Bogus Journey
Peter helps Bill Clinton with his flat tire, and they soon become friends. They begin spending a lot of time together and Lois begins to believe that Bill is inflicting bad influence on Peter. But when she confronts Bill, they end up having sex. Peter, upset that his wife would do this, tries to get back at Lois by having an affair of his own, but soon realizes that he loves her too much to do so. Meanwhile, Lois and Stewie try to potty-train Brian.