This is maybe the single most bitter and relentless squabble within poly circles: are swingers poly or not? There are a lot of points of view on the subject, of course, too many for me to quote them all. I don't think I can reasonably pretend to give an unbiased presentation of the question, but I will try in my biased way to provide a fair amount of background.
Let's assume for a moment that polyamory is defined as responsible non-monogamy (though lack of unanimity on that point probably has a lot to do with the debate). Is a swinger responsibly non-monogamous? It's not much easier to define "swinger" than it is to define "polyamory," so I'm left with my very limited knowledge on the subject: there there is a network of clubs in the world, more often referred to in their own literature as social clubs or couples' clubs than "swing clubs," where a variety of diversions go on, including private or public sex between people who didn't walk in together, and within groups. I've never been to a swing club, so I don't have much to say about the particulars, but this is the swinger world, and its population dwarfs the fledgling population identifying as poly.
A few things are generally true about swing clubs: couples are welcome, and single females are generally welcome as well. Single males are almost always barred. Bisexual behavior tends to be encouraged in females, but actively discouraged or even banned in men. A lot of the sex-play, not all of it, is between couples.
On the face of it, those who frequent these clubs are certainly not strictly monogamous, and though there might be cheaters in attendance, the norm of attending in couples presumably makes for a basic population of mutually informed and consentual players. So if responsible non-monogamy is the definition of poly, they're poly, or at least most of them are. And this, clearly, is my position--I'm happy to regard them as a subset. I don't mind saying this clearly, as so many prominent and vocal web resources crusade for their strict separation without so much as mentioning that the matter is an ongoing bone of contention. Ultimately, I think the drive to set the swingers at a great distance is assimilationism, which I regard as a misguided political strategy anyway.
The objections raised by those who distinguish between them are virtually always along the lines of "polyamory is about love, and swinging is about sex." There are sometimes more interesting points made, as there were in a recentish issue of Loving More Magazine which I seem inconveniently to have misplaced. For now, that fact will make this a shorter discussion.
After reading that article, plotting a rebuttal, I went to look at some swing-club websites to check on a couple of points. That was a lesson to me: I hadn't looked at any such sites before, and for a little while I lost my resolve. Whether swingers are poly or not, I thought, they are clearly not my kind of people. At all. More than half of the sites were full of lurid pornography ads of the most stereotypical sort, a steady stream of wickedly chauvinistic double standards, general crassness--they conveyed a worldview that's just plain ugly to me. That was news, in its way. Not very surprising news, but still something I hadn't exactly known.
Nevertheless, I read through a few, and there were some interestingly undecisive findings. Indeed, not one spoke of love--nowhere did any of them even mention the possibility that hey, you might even strike up a romance. But neither did they really play so hard on the no-strings-attached idea. Basically the idea of relationships, except for frequent (and not insignificant) mentions of friendship, is simply not addressed.
It's always a trick figuring out whose prerogative it is to define or classify a school of thought. Are Mormons Christians? I'd say yes, because unless I'm badly misinformed, they consider themselves Christians--but many Christians, maybe most, declare that they are not. Are those black separatists shouting from soapboxes in Times Square Jews? I doubt if any other Jews in the world would acknowledge them--and this time I tend to agree. (Other claims of Lost Tribehood, however, are variously more convincing to me, and meet with varied reaction from established Jews.) But when is it okay to disregard people's beliefs about themselves, and when must you take them at their word?
So. Just as I don't recognize the authority of poly purists who would cast the swingers out, I deny myself any authority to classify them with any finality. I am not a swinger myself so I shouldn't philosophize about that lifestyle. They're certainly not trying to hijack somebody else's denomination for their own political ends. They get to define themselves.
The trick, though--and, I suspect, the thing that keeps this debate hopping, and will continue to for a long time--is that most swingers, like most people at large, haven't heard of us. Of course they themselves don't claim to be a subset of polyamory. They've no clue what polyamory is. I suspect that as word gets out--which is happening faster and faster--many swingers will be just as happy with one label as the other, and some may find poly circles more congenial to their own ideas than the swing circuit they've found already. A whole lot of people might consider themselves to belong to both camps (some do already, of course, but I mean a lot).
But that, we cannot settle yet. And until the swingers catch on to us, and choose their own self-definitions one by one, the rest of us are just second-guessing.