So anyone who is not married could not know it's good unless they themselves try it?
Like I said, there are more successful open marriages than many people realize. Perhaps you thought that was just theoretical speculation on my part. No, I know of two such cases in my extended family. One of them has lasted for decades. But each case is different. The public myth of them is probably that they are wild orgies on the side. Rather, what typically happens is that one or both partners finds someone else as a close friend and long-term lover, but the marriage remains for friendship, family, legal, health-insurance, property, and other reasons. And when everyone is happy about it, they tend to keep it very low-key because many Americans somehow think it's a horrible tragedy or something.
So yes, I've seen it work, and work very well for the long term. But it's not for everyone.