Mrs. Stone's Thing (1970)

Housewife Martha wonders why her husband, George spends so much time at parties, then is all "sexed out" for days afterwards. She discovers that he's been going to swingers' orgies, and in an attempt to salvage her marriage, persuades him to take her with him to a party.. At first she doesn't care for it, but little by little she begins to get into it.