In my opinion the problem is when people within the situation are unhappy. Not necessarily all of them but at least one of them. If everyone is happy, I see no reason to comment on how they decide to lead their life and relationships.
When people are unhappy, and they say the rules make them unhappy, though, it's good to think of ways to change that. Yes, leaving is always an option, but I would hope that it would be a last resort after everyone in the relationship at least gives it a fair try to make things better.