From experience I can say that the "Best" relationship I have ever had was with a beautiful woman in which we agreed to be completely open with other sexual partners.
She would bring women home that needed some rebound sex and I either had the option to make love to them or not. We had the ménage à trois that everyone talks about and she let me pick the partner and we occasionally would sleep with other partners.
However as time went on we found we preferred each others company the best, as there would always be some residual emotions left over from the multi-partner experience that was uncomfortable and mildly painful.
The part I found most refreshing was the honesty and trust it garnered in each other as we never hid anything from each other and were able to openly talk about any topic or fantasy we might consider regardless of whether we acted upon them or not.
Had she not wanted to live in our Nation's Capitol we might still be together as I still consider her my soul mate, but having lived there for 18 years of my life I had no wish to go back and so we went our separate ways . . . however we both have acknowledged to each other that we have never found a relationship more fulfilling since!