As much as I think Charlotte is a crazy person, I think this time she is correct. The relationship Cooper has with Violet, is intimate and it is inappropriate to have an intimate relationship with a woman when you are married or in a committed relationship with someone else. It doesn't matter that it isn't sexual. Even if it was just over the internet, it still wouldn't be right. When you are married(etc..)all your intimacy is for that person, not just the physical intimacy. If Cooper wants to commit to her then the nature of his relationship with Violet has to change. It is a basic fact of relationships. You are either together 100% or don't bother.