Sexeo (1985)

Greg is a porn store's clerk who narrates through a series of events that occur when Keith takes a porn tape called Sexeo, that promises to cause his wife Celia to sleep with him. Alas, Celia finds it so mesmerizing that she shows it to her friend Monica and a lesbian encounter follows. Keith's son and daughter then find the tape and it causes them to sleep with each other. Keith's friend Robert eventually joins in. Robert then uses it with his wife Monica, who later on sleeps with Keith's son. The two couples - Keith and Celia and Robert and Monica - then conduct a private swinger party.