There are some divorces where there are no clear good guys or bad guys. Sometimes, a couple falls out of love and can't seem to repair what they once had. Or maybe they're fighting so much that they know their kid's lives would be much more stable if they broke up. When a couple calls it quits because of an affair, everybody knows who the bad guy is. The other man/woman is the secondary antagonist, and those are clearly defined roles.
That being said, just because someone ends their marriage by having an affair doesn't mean they can't make up for their misdeeds in other ways. One teenage boy whose father left his mother for his mistress asked him to financially support his mother through school, even though he wasn't legally obliged to do so. His father agreed, but the woman he married had a problem with it because of the various extremely expensive hobbies of her children.
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