Consider a traditional "save myself till I am married" person. Are they monogamous on the morning of their wedding day? They haven't "done it" yet, but I would argue they are already mono.
When a couple I knew in the late 70s got married, they refused to use the line in the service about "holding myself only unto him/her" as they were sure they would not keep to it. They were quite happy to keep in the "to death do us part" bit. I consider they were already poly-ish at that point, even tho at the time theirs was an exclusive relationship: in their heads they were already open to an open marriage.
So yes: you can be poly and be currently with only one partner, or even like myself with none.
NB: Not the same River as posted immediately before this: we have two folk on these forums with the same exquisite taste in chosen names