Okay, as a wife who opened her marriage in the wrong way, with an emotional affair, and is now looking back at how things went, and kicking myself, I have a couple of things to bear in mind.
First of all, all marriages have issues. No one teaches you how to have a relationship, how to talk, how find a way to communicate. Language is just part of it. Often what you are saying is not what is heard. Knowing what you want to communicate, and how to get rid of the assumptions you have about relationships, and learn that the ones you have are not the same assumptions as other people have, is hard!
Now, a little more than five years later, our marriage is better. We learned a lot to help our marriage. But it wasn't easy; it was rough. It still is. Yes, opening up to polyamory was an instigation to a lot of these good changes. That doesn't mean that my husband sees poly as a good thing. These were changes we needed, regardless. We just had no idea how to make them, or how communicate them well.
Maybe now Nathan is learning where the cracks are in his marriage, and how to deal with them. However, that doesn't mean he has to like that it took an outside relationship to get the information they needed to help their marriage.
Also, you seem to be falling into a pretty common newbie poly trap. Some never fall out of it, sadly. That trap is pretentiousness. You come off as very pretentious. You may think you don't, but I read it in every one of your posts, so I can't believe Nathan doesn't pick up on it. You are talking about how you have been nothing but honest, and the women in your life think you are nothing but kind and considerate. You are discussing things, and trying so hard to get him to see things in your light, obviously because it's the right light, the more evolved light!
Don't stay in that trap. The truth is, he doesn't need to see things from your point of view, anymore than you need to see things from his. If what you want is so right and honest and makes sense for you, why can't what he wants be right and honest and make sense for him?
That this sounds like you have all the answers, and are doing everything right, and just don't know why he can't get on board, seems so pretentious, and honestly, from his POV, probably a little douchey.
You want him to respect you and believe that you respect him? If this, bringing all of this up, is causing Nathan and Ruth to work on issues that have long needed that work, okay. Back off. Let them. No one wants to be told their marriage is saved or made better because someone else wants to bone their spouse. Brutal, but true.