I wouldn't say that. A mono friend of ours says he simply wouldn't have the energy or interest in trying to maintain more than one relationship. He's not mono because he thinks it's more moral or that a commitment relies on exclusivity or anything else; he simply doesn't want more than one romantic relationship to deal with at a time.
That suggests the reasons for living mono can vary the same as reasons for living poly. In the face of that, I certainly wouldn't say all mono folks view love as a zero sum game. I'd actually question the notion that most do--I suspect it's simply a structure that they find works for them most of the time. Some require the exclusivity (as does Mono), while for others it's just a way that works for them most of the time, while others view it as a moral thing, and so on.
All speculation on my part, of course.
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.