Have you ever fell for someone - maybe a crush, but lets limit it to a longer-lasting, more "meaningful" kind of feeling - while in a committed relationship - that in no way lessened or detracted for your feelings toward your current SO?
If so, that's being poly. Whether you act on it or not determnines whether or not you're "living" poly.
I have come to understand that many people fall for someone and it "flips a switch"; they no longer have meaningful attraction or interest in other potential partners. I guess that's being "mono".
I honestly thought that the former was normal and that white-knuckling your way through a relationship with all these other feelings flying around was just how it was - that's what commitment was, and it was a sacrifice everyone made in order to be with the "chosen one" as it were. I didn't know there were people for whom this was never a problem until I was introduced to poly - and came to learn that MY way of living was the unusual one (according to something I'm sure).
Once the possibility of not limiting my love arose, I have found it very difficult to concieve of living otherwise. As far as actual relationships go - well, they can be difficult, especially in such a "mono-centric" society. And the pain of loss is in no way mitigated - at least not for me - by having others (though it is nice to not go through it alone), something else I find that many "mono" types don't grasp; "but you have backups". How insulting on sooo many levels...
Anyway - if you feel you're limiting your love, you're a poly person "living mono". As I have come to see and understand it, mono's who've found their "one" don't seem to feel this way.
My loose change...