There are often going to be times in a relationship where you reach an impasse: your needs become oppositional and the actions that occur as a result of these needs hurt the other person. This is something that they cannot tolerate and nor do I think they should have to, they have to raise the issue. They have to give an ultimatum, of sorts. Now, ultimatum has such bad connotations because often, the way they are presented belittles, demonizes and rejects the right the other person has to want different things that you do.
For example, lets use a situation where a wife does not want to be part of a non monogamous relationship anymore because she is unable to feel valued whilst her partner is seeing other people. A bad way to present this would be to state that you either give up your other partners or it's over. A good way to present this would be to state that she is unable to feel valued in this type of relationship, despite any efforts he has made to reassure her of her place in his life, and she cannot continue the relationship and be happy and healthy. They both mean the same thing: I can't do this anymore, I am going to end the relationship if it doesn't stop, but one acknowledges that his needs are not inferior or "less right" than her needs. Even if they are different from the norm. It also acknowledges any efforts he has made in order to maintain their relationship.
The most important part of making this a good ultimatum rather than a bad one is if they state that they need to have non monogamous relationships to be happy and healthy, the wife cannot vilify them for having different needs. She cannot make him out to be a bad guy for not simply agreeing to do what make makes her happy. She has to accept that they want different things at this point and that is okay.