Polyamory is a Terran word that is a combination of words from two separate languages of that world (poly, meaning "many," and amor, meaning "love"). It refers to romantic and/or sexual relationships involving more than two people. It is now believed that such relationships were somewhat common during the first century of The Land's existence, however after The Fall (in LY 100), such relationships were essentially unheard of prior to LY 914, and the fact that they had once occurred on the Land was lost to the mists of time.
It must be understood that the Punishment imposed by God for the Fall of the Land's humans was an increased sense of isolation from one another, a feeling of loneliness, of not truly "belonging," or not fully understanding one another. This made it more difficult for people to form close friendships, and even harder to form intimate romantic relationships than it had been before the Fall. It is known that marriage was an institution that existed prior to the Fall (for example, that of Connor and Brigid). In hindsight, it's easy to see how the increased difficulty in feeling a strong sense of connection after the Fall made it far less likely for Landians to be able to fall in love (or at least to trust in love) with more than one person at a time. But more than that, it was revealed in 914 that the first five writers of The O'Gas intentionally removed any mentions from the Book of Connor and Brigid that would have obviated the existence of polyamory. It is now believed that one side effect of the Punishment was a feeling of shame regarding certain aspects of humanity's previous psychological state, and the writers didn't want history to remember the people of the first century as deviants. Of course, the people who were alive at the time were all aware of the existence of polyamory, but apparently felt the same shame, and so never mentioned it to succeeding generations. (It is likely that any compound families that existed prior to the Fall broke up thereafter, though how anyone chose which spouses to remain with and which to leave is a matter which cannot now even be speculated upon.)
It is also known that over the course of centuries since the Fall, Landian psychology has been very slowly reverting closer to its pre-Fall state, at least for most people. (Others seem to go in the opposite direction, feeling even greater isolation, which can require psychotherapy and possibly medication.) Many psychotherapists and philosophers believe that this trend will continue, and someday in the distant future, the Punishment enacted by God will be entirely undone, though it is unknown how long a full reversion might take. While the ease with which humans relate to one another continues to increase, social anxiety will likely remain commonplace for centuries to come. (However, as the Fall pertains only to humans, and no other intelligent races such as Elves or Merfolk, so too is social anxiety considerably less common and generally less severe in those races than in humans.)
It is unknown how many people, over the past centuries, have even been aware of the possibility of polyamory. If the idea occurred to anyone, they either never mentioned it to anyone else, or were quickly discouraged from doing so, quite possibly by any spirit-talkers from whom they may have sought guidance concerning the idea. The earliest instance now known of this question came from a spirit-talker named Therman, in LY 878 (though again, this fact did not become common knowledge until 914). Since he was a child, Therman had been best friends with a boy named Harlan and a girl named Maeve. All three of them were 25 years old when Therman and Harlan both realized they were in love with Maeve. Most details of this "love triangle" have never been made public, but it is known that Maeve came to realize she was in love with Therman, but only loved Harlan as a friend. Subsequently, Harlan moved from Plist to Monab, and Therman married Maeve in 879. However, before all was said and done, Therman sought guidance from spirits he knew. One of them explained polyamory to him, and it is now known that Therman broached the subject with Maeve and Harlan. All three of them found the concept distasteful, though they ultimately decided this owed only partly to the post-Fall nature of Landian psychology. Another part of their reaction was accounted for by an aspect of polyamory that the spirit explained: it is a sort of orientation. It comes naturally to some people, while its opposite, monogamy, comes naturally to others. The spirit also said it can be difficult, perhaps in some cases impossible, to be sure of one's own orientation in this regard, when they are the products of a society that is either unaware of the concept of polyamory, or openly opposed to it. Nevertheless, Therman, Maeve, and Harlan were apparently all monogamous in nature. And even if they weren't, Maeve was certain that her feelings for Harlan were not romantic.
In 886, Therman and Maeve had a daughter named Ginger. At some point in her youth, her parents related to her the story of what had transpired years earlier between them and their old friend Harlan, as well as introducing the concept of polyamory to her. But they also told her that Landians most likely weren't ready for it, and that it might be best if she didn't mention the idea to anyone, unless she was sure they wouldn't be disturbed by it. (But they also wanted to be sure she knew that if she ever found herself in love with more than one person, that there would be nothing wrong with engaging in a polyamorous relationship, provided the objects of her affection were all amenable.) As it happened, Ginger always believed her own orientation to be monogamous, and she never felt romantic love for more than one person at a time. So, she gave the matter little thought. However, like her father, she became a spirit-talker, and in 913, she became Protestant bishop of Plist. In 914, one of her (anonymous) parishioners came to her seeking guidance about being in love with two different people. Ginger told this parishioner what her father had told her, years ago. She also sought guidance from Maeve, who showed her the notes Therman had made of his conversation with the spirit, at the time, and told her that he had submitted the notes to then Arch-bishop Esmeralda for possible publication in the O'Gas. But According to Maeve, Esmeralda had advised Therman against publishing it, saying that Landians weren't ready for it, but they would be, someday. (Esmeralda herself apparently consulted at least one spirit about the matter before giving her answer to Therman, though she took no notes on her own conversations with either the spirit or with Therman.) Later that year, Ginger presented her father's notes to Orthodox Arch-bishop Talak Archman, who declined to publish them in the O'Gas. However, Ginger then presented the notes to Protestant Arch-bishop Virginia Regent, who agreed that Therman's notes (with an introductory statement by Ginger) should be published in the Book of Ginger.
The notes claim, as already noted, that polyamory is natural for some people, while monogamy is natural for others. They also say that God has no objections to it, and in fact thinks it's a fine thing. (Though due to His "inherent omniscience," polyamory was one of several things He had previously objected to on various other worlds, including Earth. He had changed His mind in this matter some time prior to the creation of the Land, though because He was no longer in direct communication with people on other worlds as He would be with Landians, it took longer for the people of some worlds to accept things such as polyamory on their own.) Since the publication of Therman's notes in the Protestant version of the O'Gas, many people have come to believe that polyamory makes perfect sense, while others continue to find the concept so distasteful that they believe it must be unnatural, regardless of what any spirits might say. (Some even claim that any spirits who say it's okay must be servants of Lucifer, a claim which is given no credence by most spirit-talkers, even within the Orthodoxy.) But even those who find the concept itself sound, and even pleasant in theory (including some who say they feel their own orientation would probably be polyamorous if not for being subject to the Landian psychology instigated by the Fall), tend to think it's not something they could do, themselves. It is, however, believed by an increasing number of people that polyamory will someday be commonplace on the Land, perhaps not too many generations hence. Meanwhile, if any people in the present (such as the parishioner who brought the issue up with Ginger) find themselves capable of experimentation with like-minded individuals, it is hoped that they will find acceptance from their friends and family, from the Church (whether Orthodox, Protestant, or Independent), and from society at large. (Certainly the government can have no interest in the matter, because unlike on worlds such as Earth, Landian marriages are entirely personal arrangements, with no legal recognition or restrictions.)