Open Relationships are a completely different type of relationship. One cannot compare this to marriage or intimacy in the same way we cannot compare friendship to parenthood - both may be great, but they are very different.
Open relationships work for only certain types of people. I would argue that those people fall into one of two categories:
Group 1 - is those people who cannot form meaningful pair bonds with another person. They like sex and it feels good and they want closeness, but true intimacy is a mystery to them. This can happen for a lot of different reasons, and we should not judge those reasons (because who really asks for this?) but this exists.
Group 2 - People who have been burned so badly so many times that they realize that all relationships are ultimately temporary and open anyway, so why not just make that transparent.
I am not saying I agree with this lifestyle. In fact I know that I could never even try this. I am oriented toward a 1 man-1 woman situation. But I get why people go that route. Our culture has poisoned marriage and motherhood, so that does not leave much else.