The Big Bang Theory Season 3 Episode 23 : The Lunar Excitation
The guys set up an experiment on the building's roof to bounce a laser off of the moon. Leonard goes to Penny's apartment to invite her to watch, only to find she has male company. Penny's date Zack thinks the experiment sounds cool, so he and Penny accompany Leonard to the roof, where Zack shows himself to be extraordinarily dim-witted. Later that night, Penny, realizing Leonard has made it impossible for her to enjoy dating unintelligent men, gets drunk and sleeps with Leonard. The next morning she tells Leonard their encounter was "meaningless". Leonard, now thinking it's routine for friends to have "meaningless" casual sex, asks Leslie Winkle, then Penny, to sleep with him—and is confused when they slam their doors in his face.