Iti s absolutely correct that such types of relationships/marriages are more common than maybe thought anywhere on the globe. But I think
3Diddlyf95zone ´s kicker words were "genuine" and "intimacy". How genuine is a relationship without any intimacy and I do not speak just about full sex, but also and especially kisses, hugs, a gift out of the blue, going out together, the "little" things in intimacy?
That is more a business thing than a relationship then.
An example: Friends of us are very much into swinging/sharing and they have some ironclad rules what they do and what they do not. But despite them "fucking around", there is no question they are a couple when you see them. The little intimacies and gestures make it very clear those two are happy together, no matter that maybe yesterday she got it off with 3 other guys and her husband or ladyfriends "visited" them.
Another longtime friends couple of us, are definitely not into things like sharing or so, but they were happy quite a time. Now they are on the way towards divorce, which is something we find sad, since they are good people, maybe they stay together until the kids are grown up.
But that something is wrong could be seen for at least a couple years. Nothing big, but e.g. the missing hugs, far more than usual separate activities,..., we friends could guess something was wrong. Today they are more like roommates than a couple.