Sexual intercourse between two people where one or both of those involved are married to someone else. In his humorous dictionary, The Penthouse Sexicon (1968), Frederic Mullally defines:
-- Adult: ' Person who is supposed to know what he or she is doing.'
(1) Robbie Ross (George Segal) and his ex-wife Natasha O'Brien (Jacqueline Bisset) in Who is Killing the Great Chefs of Europe? (1978): -- Robbie: ' You don't call a roll in the hay with your secretary adultery.' -- Natasha: ' What would you call it? Shorthand? '
(2)Tom Wingo (Nick Nolte) and Susan Lowenstein (Barbra Streisand) in The Prince of Tides (1991): -- Tom: ' Lowenstein, I think I like adultery.' -- Susan: ' That's why they made it a sin.'