Mile high club
The mile-high club is slang for people who have had sexual intercourse on board an aircraft during flight. While this usually refers to sexual acts in an airliner lavatory (for privacy), a sexual act anywhere on a plane in flight would fit the term. The airplane just needs to be in the air (i.e. no landing gears touching he tarmac), not literally a mile above the ground, during the sexual activity, in order to qualify for the "mile high club".
An alleged explanation for wanting to perform the act is the supposed vibration of the plane. Some also theorize that people who engage in the act have had personal fantasies about pilots, flight attendants, or other aircraft crew, 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, as well as the thrill of simply engaging in sexual intercourse thousands of feet above the Earth.