Dogging

Definition
dogging (sexual slang): British English euphemism for engaging in sexual acts in a public or semi-public place or watching others doing so.

There may be more than 2 participants; both group sex and gang banging can be included. As observation is encouraged, voyeurism and exhibitionism are closely associated with dogging. The two sets of people involved often meet either randomly or (increasingly) arrange to meet up beforehand over the Internet.