People also promise to love and honor each other (until death...), and see how well that's working out for many. Cheating is often only the consequence of that failure, and as someone "outside" - who are we to judge the people who are inside of the relationship?

Only some religious traditions actually involve that oath. Jews have practiced a religious version of no-fault divorce for thousands of years.

Or we just culturally decided that a centuries-old tradition didn’t mean we had to forever put up with something we only later found out we’re unhappy with. Marriage should not be a life sentence nor an unbreakable commitment.

Funny things, the rules we make up to justify nonsensical behaviour.