The 'ability' theory use to be my definition, but experience has changed that.
I would like to believe, that people are poly regardless of their relationship status, but I think the actuality is different.
For instance, if you have a poly person talking to a 'open-minded' couple who don't currently have outside partners, odds are, the poly person will refer to the couple as 'living monogamously.' Sometimes, even regardless of the intention.
There does seem to be a line drawn in most scenarios.
If you are at least 'looking' with serious intent, then you get the poly label.
If not, it seems most people might refer to someone as open-minded, if they are single, or 'living monogamously' if they are a couple.
So we are defined (by others) according to our relationship structure in most cases.
Now we could debate if that is just a old, ingrained-habit or not,..but that is for a different thread.