The mile high club is a slang term applied collectively to individuals who have engaged in sexual intercourse while on board an aircraft in flight at least 1 mile above the Earth.
Some people attribute the allure of the club to the lower atmospheric pressure in the flight cabin, which they claim increases the intensity of orgasms. Another explanation is the vibration of the airplane, which may make arousal easier. Others say they have fantasies about pilots or flight attendants, or a fetish about planes themselves.
For many others, perhaps the majority, the appeal of joining the MHC is the thrill of doing something taboo and the thrill of the risk of being discovered.