I know of couples who have had various types of "agreements". Some of these have worked for a period of time. What seems to have happened is that - 1. one or other met someone who they really liked and eventually left the primary relationship (which may well have been why they opened up the relationship in the first place). 2. Either one or both got rid of the "itch" and over time couldn't really be bothered with anyone else.
Sorry, probably not helpful. I think these type of arrangment tend to be bridging.
I would google it - under polyamory or something like that. I am sure that there are people who have made it work as well as a traditional marriage (which let's face it doesn't work that well a lot of the time).
Who was that aristo bloke who had all the "wifelets" - they seemed to put up with it, but then he was extremely rich.
I can tell you one thing - if my partner refused to ever have any physical contact of any sort with me - I would most definitely consider it my right to go elsewhere. And why should I necessarily have to go through the hassle and trauma (for me and the children) of a divorce and giving up a nice comfortable home. That would make me look like the bad guy. So I would have no qualms at all about getting my needs met outside the marriage.
I have a few friends whose long term partners no longer want to any kind of physical intimacy - I think they are completely mad not to get their needs met elsewhere.