Here's a quote:
The original Twitter discussion [that prompted the article] was about love triangles in YA fiction (love ‘em or hate ‘em?), which spurred me to think about the trope as a whole: why does it have to be combative? So many books use the triangle to push plot but would never consider letting the three characters in question come together. [more...]
It was a curious experience for me to read this article by Ms. Mandelo a day after re-reading the Song of Songs, which is Solomon's. It is as if I stepped from one universe into another.
I'm not going to focus on the moral issues involved here, particularly the issue of depicting kinky relationships in stories aimed at older children (what the "YA" label actually means), because covering that thoroughly would require a great deal of time and space and would carry the conversation into a realm different from the one I'm interested in at the moment. Instead, I'm going to focus more heavily on the artistic matters.
The brief answer to the question Ms. Mandelo poses is, because ending a love triangle with a three-way destroys the plot (or subplot) and betrays the reader.
Love triangles are of perpetual interest and popularity in fiction for at least two reasons: first, they touch on common human experience with which a large number of readers can relate, and second, they produce tension by creating a situation--a competition of sorts--that at least one character must inevitably lose. The build-up in that tension, with all the opportunities for shippers to argue rancorously on the Internet over who should lose and who should win, creates the pleasure of reading about love triangles. Twilight
Take any generic Japanese harem comedy as a further example. Basically an exaggerated love triangle, the harem comedy is a story in which three to seven beautiful but eccentric girls are trying for improbable reasons to latch onto one erstwhile luckless loser. The harem comedy is interesting and entertaining precisely because only one girl can get the guy in the end; if it were a literal harem rather than a harem comedy, there would be no plot. If the story ended with the characters deciding to have a four-way, or an eight-way, it might be kinky, but it would be a dissolution rather than a resolution to the storyline, because the storyline is about one girl getting a guy--or perhaps more accurately, one guy choosing the right girl--and to end the story in such a way that the difficult choice does not have to be made, is cheating. It is a letdown. It is anticlimactic. This is because harem comedies, although they have a tendency to get mired in anatomical gags and sexual hijinks, are when stripped bare (so to speak) basically boy-meets-girl stories. They are love stories. And love stories are by nature exclusive and monogamous. More on that a little later.
Take Ranma 1/2
When I think of stories containing relationships that might be called polyamorous, the first thing that springs to my mind is Paint Your Wagon
I am convinced romance is at its heart monogamous. This would seem to be best in tune with our biology; romance is about sex, and sex is our method of reproduction, and sexual reproduction can only occur between two people. But besides this, romantic desire itself seems to have something of exclusivity about it. The aforementioned Song of Songs, written in a society where polygyny was generally accepted, necessarily has a monogamous bent when it describes romantic love:
As a lily among brambles,
so is my love among maidens.
And again:
My dove, my perfect one, is the only one,
the darling of her mother,
flawless to her that bore her. (NRSV)
Romantic passion inspires vows of fidelity and exclusivity. When lovers describe what they love about one another, they emphasize uniqueness.
I am not claiming, of course, that only chaste stories leading to happy marriages can make moving love stories. I am much moved by the story of Abelard and Heloise, a real-life tragic tale as full of melodrama as any bodice-ripper, but which is basically about a tutor fornicating with his student. I am also much moved by the story of Tristam and Isolde, which is about a knight committing adultery with his lord's wife. I am moved, too, by Romeo and Juliet
But compare those, which are about exclusive love (even the ones with complications like unwanted marriages), to, say, Stranger in a Strange Land
This is a pristine example of exactly what I was talking about above: to conclude a love triangle in this way is to destroy the plot and betray the reader. In this case, Heinlein did it on purpose as a sort of twist, but the story that develops from there, whatever else it may be, is decidedly not a love story. It is more like an anti-love story: it makes the claim that life would be happier if all the things that go into love stories were done away.
Some time ago, I had in my possession an interview with sf author Nalo Hopkinson featured in the Novel and Short Story Writer's Market
I don't know what this business is about rooms and times of day, and I note, looking at the news articles about obesity, that casually eating what we want when we want where we want with whom we want hasn't exactly been the best thing for our health, but that doesn't matter because the analogy is invalid; eating, at least when engaged in rightly, is a pleasurable activity that keeps the human body in good health, whereas sex is a pleasurable activity that produces other human beings and so should be expected to be freighted with peculiar moral obligations and maybe even some mystical mummery. But more importantly for this particular essay, Hopkinson's depiction of sex is decidedly unromantic. Good sex, as she depicts it in this interview, is the orgasmic equivalent of junk food--it is utterly casual, and once again the identity of the other person is of little importance.
So, to the question of why more love triangles don't end in three-way polyamorous relationships, the answer is, because that is not a love triangle.