They think because they're not emotionally involved, it's not considered cheating and sure, IF both parties consent to see other people while together or at least both know that they have that option and don't mind. But they are married and doesn't matter if it is political or don't like each other. The spouse deserves the same respect. And ok, even if he married the wrong person, how can he behave like that in front of the BROTHER of the girl he said he loves. That's just bad taste.
They just want to like the ML because he's 'hot' and has a sad past.
Or maybe they're too forgiving and don't care about morales lol

Cheating in chapter 25