Mr. Ivory here. I keep hearing the phrase "equal love" here, and I don't think that's the way most poly people think about love. In order for me to be poly, I don't have to be in a romantic relationship with someone other than Mrs. Ivory and whom I love as much as I love Mrs. Ivory. That's crazy. Every love relationship is different, and no level of love is more or less valid than any other.
The defining characteristic of poly is simply that poly couples allow for the possibility of multiple love relationships beyond the primary one. These relationships may or may not include sex. This, I think, answers Chicup's question about the difference between a long-term swing partner and a poly secondary, teritary, etc.
I think a lot of people accept the mono-normative notion that one can only really have one "true" love, and that one person must devote all of his love to that one person in order for it to be "real" love. So the idea becomes that loving another person must diminish the love one has for his primary. But we don't apply this standard to other kinds of love. If a couple has a second child, they don't have to tap into the limited pool of love for the first child and fish out some love for the second (thereby loving the first child less). They just manufacture some more love.
Poly is the same way. I love Mrs. Ivory. If I meet someone else whom I also love, then I'll create some more love. I'll still love Mrs. Ivory just as much. I'll just love that other person as well. Yes, I'm capable of that much love. Understanding this concept is key to understanding poly.