The mile-high club is slang for the people who have had sexual intercourse on board an aircraft in flight.
An alleged explanation for wanting to perform the act is the supposed vibration of the plane. Some say they have fantasies about pilots or flight attendants, or a fetish about planes themselves, a type of mechanophilia. For others, the appeal of joining the mile-high club is the thrill of doing something taboo and the thrill of the risk of being discovered.