The mile high club is slang term applied to individuals who have engaged in sexual activity while on board an aircraft. There is no known formally constituted club so named.
(According to the BBC, travel lawyer Philip Banks, from the firm Irwin Mitchell, says that getting frisky in the toilet of a plane could be a criminal offence.
Seemingly a person has committed an offence if they have sex in a lavatory to which the public has access, under section 71 of the Sexual Offences Act 2004. You have been warned.)