The Codpiece Topology
The Big Bang Theory Season 2 Episode 2 (09/29/08)
The guys return from the Renaissance fair and see Penny has already moved on to a new guy, Eric (Travis Schuldt), prompting Leonard to pursue a relationship with prior fling Leslie Winkle.
Leslie agrees Leonard, who kicks Sheldon out of their apartment for their date. Sheldon moves out to the stairwell to play Super Mario 64 where he explains to Penny how angry he is that Leonard is dating his nemesis, Sheldon.
Sheldon returns to their apartment and tells them he formally approves of their relationship, overlooking some of her physics beliefs. When Leonard takes Sheldon's side and not Leslie's, she sees it as a deal breaker and ends their relationship.