I don't know how this relates to poly relationships, but it seems to me that most monogamous relationships start with a big, NRE-based sexual bang, and then settle into sort of a 'routine sex' situation. Where you typically desire sex much less than initially, but you still do it fairly regularly, although nobody enjoys it all that much. That yearning? I suspect its fairly rare in mature relationships.
That doesn't mean the relationship is no good, imo, just means its in a different place. Less sex, but often much more depth, in that you know and love each other so well.
But only you, and time, can answer whether you're really mono and done with DH romantically, or poly and having bigtime NRE.