They aren't couples any more.
I’ve seen two couples go the ‘open marriage’ route, one back in the 70s and one just recently. Those ‘marriages’, if that’s what they can be called, blew up in short order.
In both cases the guilty parties insisted that they would never suffer from jealousy and other such primitive emotions. No, they were an advanced form of human being and above all that. And of course the idea of sin they found utterly ridiculous. I never did get to hear their explanations for what exactly went wrong with their grand vision, but it probably would be about as foolish as their initial explanation for why it was a Good Idea in the first place.