Curly and I figured out early on that we weren't interested in hunting unicorns. Yeah, there were some idle fantasies about possibly meeting somebody who rocked both of our worlds; though never any serious thought.
I know that at least one of the ladies I've dated thought the very idea of getting involved with Curly in addition to me just really creepy. I'll posit that as the other end of the spectrum from your experience.
I found myself agreeing with that notion quite strongly, actually. Faced with the possibility of that actually happening--a bisexual wife and a bisexual amorata--I was distinctly uncomfortable with the thought. I don't think I'd be comfortable with that, when it comes down to it.
I have wondered what it would be like to get involved with a couple of ladies I know who are involved with each other. I never thought of it as getting involved with a couple, though, so I'm not certain the experience is the same as yours.
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.