Uh... no it's not. You are either faithful, or your not.
I love advice on 'how things are out there' from people who make their living pretending to be other people. How detached from reality can one possibly be?
Holly-weird folks must be space aliens.
Well, I think it's mostly that they are economically insulated from the usual consequences of divorce. The kids are going to the same private school, regardless. Both partners will still be living in the same expensive houses, regardless. Both partners will still be sleeping with the same people, regardless. ;)
So monogamy, strictly speaking, isn't really of any importance in their lives. Once people get to a point where they aren't liking each other much, the "Hey, I'm the star, here!" attitude kicks in, and they decide to do whatever they want.