Spectators (1984)

Kay and Paul have become dissatisfied with their sex life, he moreso than she. At Paul's insistence, Kay agrees to bring others into their bed, but all Paul wants to do is watch Kay with the others, then have sex with her afterwards, one-on-one. After a series of couplings, triplings and otherings, Paul ties Kay up and gags her and forces her to watch other people going at it. This proves too much for Kay and she finally decides to leave Paul.