"Ugly and infantile" describes the ex's and two mistresses' behavior. If Kate, the protagonist of the story, only wishes to confront and not cause permanant damage to the three of them, I would commend her, not condemn her for it. Even for women who have 'moved on' past the adulterous husband and his behavior, there is often still a little satisfaction in seeing everyone involved getting a small dose of karma from time to time.