I personally believe that people are polyamorous rather than relationships. To me polyamory is the ABILITY to love more than one person.
Monogamy on the other hand, imo at least, describes a relationship (mono - one, gamos - marriage in origin). Non-monogamy would therefore describe all OTHER relationships in their various forms: open relationships/marriage, triad, quad, multiple LTRs. I believe polyamory is the trait that is applied to humans, and non-monogamy is a better term for the diverse collection of relationship styles that we refer to as "polyamory" on this site.
Pure semantics anyway, like you said
I think that polyamory is like sexuality in this case - you don't actually have to have sex with the opposite gender (straight), same gender (gay) or both genders (bi) to be that orientation, it's a self-definition. Likewise you don't actually have to be in multiple relationships, or be in love with two (or more) people to BE polyamorous.