Excuse me, while I slip into my Rant Costume…
“really cool girlfriend”? BY WHOSE STANDARDS?
A sexual relationship outside of your main ones is not a “cool” thing, like a haircut, or a new blouse!
gibber foam growl
Not at you, mind you. That’s directed more at the guys (who are, much to my regret, not hypothetical) who think that a threesome would be “cool.” You’re interacting with people’s bodies, minds, and hearts here.
goes off in corner to glower at the universe
I’m much in agreement, but I think that being wired exclusively for polyamory is likely to be a combination of a large number of factors rather than something simple. I know people for whom their exclusive wiring is such that they can’t have a healthy closed relationship; I know people for whom their exclusive wiring is such that they need to have a certain number of active relationships (sometimes of a given type) for any of them to be healthy. These strike me as very different orientations.
And then there are people who don’t care about the superstructure, and the people who don’t care about the superstructure except that if it’s one way it has to have certain traits too. (My husband’s one of the last, as a for-example – he’s happy with a monogamous relationship structure and happy with a poly one, but if the structure is poly it has to be open for him to find it functional. Since I won’t do monogamy and don’t care about open/closed, we don’t have any “no more relationships” rules.)
I don’t tend to care about whether people are on the same part of the spectrum so much as caring whether their particular set of relationship needs matches what I have available to offer them. Someone who expects monogamy of me is out of luck; with someone who is entirely content being monogamous to someone who isn’t, on the other hand, there’s the possibility of a perfectly satisfactory relationship setup.
So in answer to my OP, it seems the lines are pretty cleanly split between “sure, why not?” and “Are you CRAZY?!” 
I’m seriously wondering if we’re seeing the beginnings of a speciation based on sexual selection: monogamous and polyamorous sub-species.