Confessions of a Young American Housewife (1974)

Young, happily-married New Yorker Carole enjoys being part of a swinging foursome with her husband Eddie and their friend-couple Anna and Pete. This serves to keep things interesting. One day her widowed mother Jennifer comes to stay in their house, but she is more on the conservative side and doesn't know about her partner-swapping daughter. Soon she herself kicks over the traces and starts to enjoy the favors of both male and female bed-mates.