1x13 "The One with the Boobies"

After Chandler accidentally sees Rachel wandering around topless after her shower, a chain reaction of nudity ensues when she tries to even the score by seeing him naked. She sees Joey naked, who then sees Monica naked, who then sees Joey's father naked. Joey finds himself playing parent when he has to decide whether to hide his father's affair from his mom. And no one can stand Phoebe's new boyfriend, an irritating psychologist named Roger (Fisher Stevens).