Keep It in the Family (1973)

Young adults Alex McDonald and Karen Sayers are living in a communal situation. Falling in love, they feel they've outgrown their current situation and want a place of their own. Unable to afford the apartment they want, they ask their respective parents for the money for at least the first month's rent. Their parents refuse. As they figure their parents have the economic power, Alex and Karen decide to get back at their parents with the only power they think they have. They decide literally and figuratively to screw their parents by sexual manipulation: Karen will seduce Alex's father Roy, and Alex will seduce Karen's mother Celia, despite each parent still being married. Alex and Karen's initial "in" is through work where Karen gets a job working as Roy's secretary in his law firm, and Alex gets a job working as Celia's gardener. The respective seductions are easy and successful. As time goes on, Alex becomes jealous of Karen's relationship with his father. Karen and Alex break up because of this situation. Roy and Karen ultimately decide to get married. Complications ensue when both sets of families end up meeting each other prior to the wedding, and each individual comes to their own conclusion with whom they're really in love.