I'm thinking that perhaps revisiting the Greek and Latin may prove fruitful--and have new terms tapping the same language sources. Would make for a bit of linguistic consistency.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.