Being a bit cautious of the term "principles" we'll try to just put out one or two "concepts" that might be seen as some type of dividing lines between poly & mono outlooks with the required disclaimer IOHO (In Our Humble Opinion)
1> In poly-minded folk, there's an acceptance of the fact/possibility that it's possible to have deep feelings/ caring/emotional bonding (love?) about someone other than one's primary mate and that the existence of this is not viewed as a threat to the primary relationship.
2> The expression of this caring in a sexual manner is accepted when it leads in that direction. It doesn't always, nor is it a requirement, but if it does it's just acknowledged as one component of a close relationship.
It would seem that these basic tenets seem to be the line in the sand in most cases - IOHO
As a sidenote we saw a comment somewhere in a forum or somewhere this morning stating that a mono relationship was much more complicated to maneuver than a poly relationship. LOVE to know how that conclusion got drawn <chuckle>