Sedoretu is a fictional marriage structure invented by American speculative fiction author Ursula K. LeGuin, and depicted in three short stories set on the fictional planet O, occupied by the human race called the ki'O. O is described to be four light years from the planet Hain, which places these stories within the same canon as LeGuin's Hainish Cycle novels, which include her most famous works, The Left Hand of Darkness, The Dispossessed, and Rocannon's World.
In sedoretu, two men and two women marry each other in a bisexual, polyamorous foursome, with the members of the foursome selected from either of two "moieties," social divisions which are used to prevent inbreeding among the small rural populations which make up most of O. The moieties of O are called the "Morning people" and the "Evening people," and each moiety is believed to make up approximately half the population of O, evenly dispersed among all its communities. Members of the same moiety may not have sexual interactions with one another.
"A Fisherman of the Inland Sea," also titled "Another Story," in the 1994 short story collection of the same title, describes the sedoretu, as though to an alien visitor:
"A ki’O marriage, called a sedoretu, consists of a Morning woman and man and an Evening woman and man; the heterosexual pairs are called Morning and Evening according to the woman’s moiety; the homosexual pairs are called Day —the two women—and Night—the two men."
The narrator goes on to explain how central to ki'O culture the matter of sedoretu is:
"So rigidly structured a marriage, where each of four people must be sexually compatible with two of the others while never having sex with the fourth— clearly this takes some arranging. Making sedoretu is a major occupation of my people. Experimenting is encouraged; foursomes form and dissolve, couples 'try on' other couples, mixing and matching. Brokers, traditionally elderly widowers, go about among the farmholds of the dispersed villages, arranging meetings, setting up field dances, serving as universal confidants. Many marriages begin as a love match of one couple, either homosexual or heterosexual, to which another pair or two separate people become attached. Many marriages are brokered or arranged by the village elders from beginning to end."
"Another Story" also describes the kinship terms used in sedoretu, with regard to parents and children. A child's non-biological parent is their "othermother" or "otherfather." Furthermore:
"Many people attach themselves to a brother’s or sister’s marriage as aunt or uncle, a position with limited, clearly defined responsibilities; they can have sex with either or both spouses of the other moiety, thus sometimes increasing the sedoretu from four to seven or eight. Children of that relationship are called cousins. The children of one mother are brothers or sisters to one another; the children of the Morning and the children of the Evening are germanes. Brothers, sisters, and first cousins may not marry, but germanes may. In some less conservative parts of O germane marriages are looked at askance, but they are common and respected in my region."
The reader is given to understand, through this statement, that even in a conventional four-party sedoretu, it is unacceptable for a pair of same-moiety siblings to marry each other, regardless that they will not have sex or produce children together, but it is fully acceptable for siblings to join an existing sedoretu, extending its membership beyond four parties.
In ki'O farmholds - the typical social unit - there are hierarchies of "First Sedoretu," "Second Sedoretu," and so on, referring to the chronological order in which they were formed, and usually also referring to the ages and generations of the respective relationships. A farmhold's first sedoretu conducts most official business on behalf of the farmhold, with outside groups and other farmholds, including helping negotiate younger sedoretus. The second sedoretu conducts most internal business, such as managing logistics of resources and appointing who will conduct specific categories of work.
Le Guin explains, in "Mountain Ways," a short story in her collection The Birthday of the World and Other Stories, how a ki'O honeymoon is conducted:
“The conduct of a new sedoretu is to some extent, and wisely, prescribed by custom and sanctioned by religion. The first night after the ceremony of marriage belongs to the Morning and Evening couples; the second night to the Day and Night couples. Thereafter the four spouses may join as and when they please, but always and only by invitation given and accepted, and the arrangements are to be known to all four. Four souls and bodies and all the years of their four lives to come are in the balance in each of those decisions and invitations; passion, negative and positive, must find its channels, and trust must be established, lest the whole structure fail to found itself solidly, or destroy itself in selfishness and jealousy and grief.”
"Unchosen Love," another short story in The Birthday of the World, explains that the moieties of O are matrilineal: all individuals are members of the same moiety as their biological mother. Sexual interaction between members of the same moiety is strictly taboo, comparable to incest, or to how cultures opposed to same-sex romance and marriage tend to perceive such relationships. It is implied strongly, but nowhere stated, that same-moiety sexual relationships are illegal, and that attempts to form a sedoretu with more than two members of the same moiety would not be legally recognised as marriage. Ki'O who wish to have sex within their own moiety, and those who wish to have marriages of fewer than four parties structured in a sedoretu, typically expatriate from Ki'O and move to another planet, usually nearby Hain, according to the narrator of "Another Story." Furthermore, visitors to O from other planets are assigned a moiety on their arrival if (and only if) they declare the intention to marry or conduct romantic relationships while on O. It is explicitly stated in "Unchosen Love," as well as in "Another Story," that the membership of moieties is obvious to the ki'O, in a similar capacity to how humans on Earth usually regard a person's gender to be obvious, not requiring to be stated aloud. It is, however, never stated how this recognition is accomplished, and it is not obvious to people from planets other than O. The narrator of "Another Story" also declares that one's own moiety is felt with as much personal depth and sense of rigid reality, as one's gender, with regard to how it integrates into their overall identity.
In "Mountain Ways," it is shown that sedoretu comes with a strong, socially pressured expectation that there be exactly two men and two women in every sedoretu, and that arrangements with other gender proportions are seen as - at minimum - contrary to convention, and possibly taboo, but far less taboo than sex within one's own moiety. The reasons for this lesser taboo appear to be a concern for the moiety distribution of the population of O: if only one couple within a sedoretu has children, then that increases the number of people in one moiety, without balancing it in the other moiety. It is implied but not explicated that sedoretu with 3:1 and 4:0 gender "imbalances" are expected to adopt orphans into their family (and moieties) as a workaround for this issue, if they wish to avoid unpleasant scrutiny or the legal invalidation of their marriage. On O, to be strictly lesbian, gay, straight, and/or monogamous, are seen as just as "queer" or even pathological, as conservative cultures on Earth consider lesbian, gay, and bisexual people to be; likewise, bisexuality and polyamory are regarded as the conventional and normal orientation and relationship framework on O, in the same way that heterosexual monogamy is regarded in modern conservative Earthling cultures to be the assumed and correct orientation.
Fanfiction has made some use of sedoretu as a framework for exploring multiple character relationships in bodies of media which have several obvious possible polyamorous dynamics. Fan writers for Buffy the Vampire Slayer, Stargate Atlantis, the BBC's Merlin and Doctor Who, the Nintendo video game Fire Emblem: Three Houses, the Marvel Cinematic Universe, and the webcomic Homestuck, have written sedoretu fanfiction at the time of this writeup.
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