The Dark Side of Tomorrow (1970)

While their husbands are away on work trips, 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.