The Dark Side of Tomorrow (1970)

While their husbands are away on a hunting trip, two bored housewives get together to commiserate. One thing leads to another, and they wind up in bed. For one of the women the incident was just a pleasant diversion, but for the other it's turned into a fixation, and when she sees her "lover" going after one of the male neighborhood hunks, things take a turn for the worse.