Forgive me if this has been mentioned, but there is a natural ebb & flow to every kind of relationship. Some people don't have the stick-to-it-ness that is required for a long term relationship. Some people quit when it gets tough. Some people secretly "strategically cheat" during those ebbs. I know of some situations where the partner had no idea it ever happened & they stayed married & always thought the other was happy in it.
There are so many variables to consider that I'm not sure it's possibly to quantify an answer here.
The more important question, I think, is: Would it work for YOU?
Steph: Wife, Bisexual, 24 year old mom to 1
Patrick: Husband, Straight, 23 year old dad to 1
Currently exploring Poly together, our latest adventure.