The Big Bang Theory Episode 5.13 The Recombination Hypothesis

Leonard decides to ask Penny out on a date, and they both agree to give their relationship another chance. They argue over dinner and remember why they didn't work in the first place, but they end up sleeping together afterwards anyway. They agree to secretly continue seeing each other under the condition that they act like friends around everyone else, just so that they can make sure the relationship works. Eventually it is revealed that this is just a fantasy for Leonard, but it does inspire him to actually ask Penny out. She agrees to go to dinner with him, and their relationship begins a new.