When Fred and Barney get tuck themselves in their bunk bed, Gazoo suddenly appears and startles them. It startles Barney so much that he falls out of his top bunk and he lands on top of Fred, making Gazoo believe they're mating (doesn't help that Gazoo's alien race has no females and they reproduce like amoebas). Talk about a gay old time!
Fred: What are you doing here? Gazoo: I'm here to experience your species' mating rituals, so get to it! Fred: Oh, no, Barney and me don't, uh... Get off of me! (shoves Barney off his back and onto the floor)