Taboo III (1984)

With only her younger son Jimmy who's more concerned with his band and best friend Blake than his mother, to keep her company, Barbara finds a friend in Joyce, whom she discovers is having an affair with her own son, Brian. Whereas Joyce and Brian have fully embraced their incestuous relationship, Barbara finds herself possessively jealous of Jimmy when she spies him with his needy and manipulative groupie girlfriend Diane. Although she is shocked to discover confidante Joyce in bed with Brian, Joyce encourages her not only to seek comfort with him but also to branch out to young guys in general starting with the pizza delivery boy. Jimmy and Brian are both helping prepare a fledgling band for a major tour and record deal, and accomplish the latter by throwing an orgy for the record company executive. After a hot night of passion with Joyce, Jimmy decides to give in and seduce his mother.