In my experience, the uniqueness thing is important
I think this conversation (won't call it discussion) is very good and very interesting, and about something many, many feel. Whether they identify as poly or not.
Often, it is the emotional closesness part that is decisive, but, as mentioned here, it may be the combination of emotional and sexual intimacy that can pose the greatest challenges. It's something like: "If you have all this with someone else, what is left for us then?"
And this goes well beyond NRE - in fact, I think it may even be dangerous to assume, at first, that it is mostly NRE. Only to find out that rather than cooling down as energy stabilizes, it gets worse.
What I have found helpful, is creating a concrete experience of uniqueness. Not only as something we talk about, but something we do. So that each partner not only knows he/she is unique in an abstract way, but also concretely: This is our activity, this is our place, this is our game. This may seem illusionary to some, but I find it is real, totally real, and more important than one might imagine. And something to be respected.