The question as to whether polyamory is a choice is an interesting one for many reasons. One of the main reasons I find the question interesting is that it relates so closely to the question as to whether homosexuality, or bisexuality, or heterosexuality for that matter, is a choice. These latter are widely understood to be "sexual orientations," and polyamory isn't considered to be a sexual orientation so much as a ... well, a "lovestyle" choice.
As a bisexual man who has had only two enduring loving relationships (six years, and then thirteeen years in duration), both with men (I had a girlfriend once, but only for a short while), I can say that I'm pretty familiar with the insides and outside, the whole terrain, of the sexual orientation choice questions and debates. Supposedly, the fact that I and my queer brothers and sisters never chose to be queer/bisexual/whatever, provides me/us with a certain kind of political clout and power that we'd not have if we somehow "chose" to be as we are. Our claim to continuity with the various great civil rights movements is supposed to be strenghthened by the fact that we no more chose our sexual orientation than black people chose the color of their skin, or women their gender, etc. This lack of choice is true, of course, but it is strange that we've been boxed in to this frame on things. And now that polyamorists are wondering among themselves if they, too, were "simply born this way", whether their "lovestyle choice" may be no more a choice than eye or hair color, I think some of the weirdness of these choice debates and frames begins to rise to the surface.
Be clear of one thing: polyamory is a cultural creation, not a biological essence. Polyamory is more like Permaculture [http://en.wikipedia.org/wiki/Permaculture]
than it is like our desire or ability to love more than one person simultaniously. Even the concept of homosexuality and heterosexuality, and bisexuality, are cultural abstractions, as is the notion of "gayness" and "queerness". But you have to look very close to see culture at work in these terms and notions. Example: the term "homosexuality" emphasizes sexuality, which is but one part or dimention of being "queer". Notice they (and "they" were the "heterosexual" coiners and populizers of the term) didn't call us queers "homoaffectionate," "homophiles".... To call someone "homosexual," or even "heterosexual" is to define an entire person around, really, an absurd abstraction--a distortion of perception about what that person is: whole.
Engaging in "nature vs nurture" debates or discussions vis-a-vis polyamory may be an interesting and playful pastime, but it isn't all that damn important, I think, either way. The discussion arises only because there is prejudice directed at those who choose this lovestyle -- Did I just say "choose"? Oh, my!
I'd rather undermine the prejudice that gives rise to the defensive posturing and maneuvering which begins with "this isn't a choice". Of course it isn't a choice! But so what? Who ever chose their desires? We can, within limits, choose what not
to desire. Vegitarians have often done that. They may really desire meat, but don't desire the torment most "farm" animals undergo in today's industrial hell-hole meat-growing operations. Eventually, this becomes seamless and the desire for meat falls away on ethical grounds.
Polyamorists are like vegitarains in this respect, in that we choose not to "cheat," lie, sneak around -- on ethical grounds, but we also choose not to be ashamed or even afraid about loving multiply. We choose to question the nearly ubiquitous assumption mantras our cultural monogamism chants to us everywhere, in movies and songs and pulpits. We even doubt the core premise of monogamism, that loving only one person at a time is superior to loving two, or three.
Permaculturists doubt that dramatically unsustainable systems in agriculture, transportation, housing..., can lead to anything other than global disaster with so many billions of people engaging in them. They choose to try to live true to life, and more ethically. Who would fault them for that?
[Note: the Wikipedia page link, for Permaculture, didn't work for me. I have no idea why!]