“I still don’t quite understand why they’re cheating, apart from habit and validation.”
Habit and validation; those are the reasons. I am not sure what more you are looking for out of that or if you feel the writers owed YOU more reasons or reasoning other than that for their affair (although it wasn’t always an affair). Not everyone has a long list of reasons to be understood for anything that they do.
If you do feel that way, then I wouldn’t trust a review from you about anything.