Bitter is one of the components of taste, in a literal sense, and one of the seasonings of living, in a metaphorical sense. Is there anything more bittersweet than a love one cannot obey?
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.