Seinfeld: The Switch
Jerry is dating a girl but really wants to date her roommate. George suggests that the only way to make the switch is to propose a menage a trois to his current girlfriend, which will turn her off and her roommate on. Jerry follows through on George's plan, and finds that both girls are "into it." But Jerry can't follow through—and George can't believe it. To Jerry, the roommates are substitutes; to George, they are complements.