@YouAreNotBatman
To be clear I’m a huge believer on everyone finding their own lifestyle, but I thought love (not sex / kids / co-dependency / social norms) was the reason why people want a relationship in the first place.
I think it certainly is what drives most people into a relationship in the first place, yes. But the "love" that people experience with a new partner is not necessarily durable. It gets sorely tested by time, by temptation, by the arrival of children and the day to day stresses of cohabitation.
Many people are able to see past this and cherish their spouses in bad times and good but for many people a relationship built 20 years previously (and basically founded upon lust with a dash of idealism thrown in) outlives its usefulness. People change, they want different things out of life as they get older.
Remaining with someone who doesn't bring out the best in you because they made you happy two decades ago isn't necessarily a recipe for optimal happiness and there's no particular nobility or moral advantage in flogging a dead horse.
Of course when there are children involved, there's a responsibility not to discard something carelessly and to make any change with great sensitivity. But this doesn't mean that lifelong monogamy should be everyone's goal.
It does mean, in my view, that people should not mislead one another and should end a relationship before beginning a new one. But we all know that life sometimes makes this extremely difficult and painful and a lot of people cheat because they convince themselves they won't get caught.
I'm not defending this: it's extremely painful and destructive to be on the receiving end of this. But it's also intensely human and the moral outrage on display here about this is fairly childish.