Sedoretus

Sedoretus are described in the fanlore.org page as thus:

Note for readers unfamiliar with the planet O:

Ki'O society is divided into two halves or moieties, called (for ancient religious reasons) the Morning and the Evening. You belong to your mother's moiety, and you can't have sex with anybody of your moiety.

Marriage on O is a foursome, the sedoretu — a man and a woman from the Morning moiety and a man and a woman from the Evening moiety. You're expected to have sex with both your spouses of the other moiety, and not to have sex with your spouse of your own moiety. So each sedoretu has two expected heterosexual relationships, two expected homosexual relationships, and two forbidden heterosexual relationships.

The expected relationships within each sedoretu are:

The Morning woman and the Evening man (the "Morning marriage")

The Evening woman and the Morning man (the "Evening marriage")

The Morning woman and the Evening woman (the "Day marriage")

The Morning man and the Evening man (the "Night marriage")

The forbidden relationships are between the Morning woman and the Morning man, and between the Evening woman and the Evening man, and they aren't called anything, except sacrilege.

It's just as complicated as it sounds, but aren't most marriages?^[1]

But it's not really that complicated! Let's use the Homestuck kids as an example.

Jade and John are ecto-related, and therefore their marriage would be forbidden. To make sure they don't accidentally marry, Jade and John decide to add a label to their names, and call themselves John Day and Jade Day. 

Rose and Dave are ecto-related, and therefore their relationship would be forbidden. To make sure they don't accidentally marry, Rose and Dave decide to add a label to their names, and call themselves with the surnames Rose Night and Dave Night.

Jade looks in the future and sees Jake, who is John's descendant, and Jane, who is Jade's descendant, though their exact family tree is unclear. To encourage her descendants to not het-marry each other, Jade passes down her surname. Jane is now surnamed Jane Day. John does the same, and Jake is now surnamed Jake Day. Together, they are Jade Day, John Day, Jane Day, and Jake Day.

Rose and Dave think this is a good idea and do the same. Their descendants Roxy and Dirk are now surnamed Roxy Night and Dirk Night. Together, they are Rose Night, Dave Night, Roxy Night, and Dirk Night.

Now the rule is "don't marry anyone who has the same surname as you", a very simple idea to transmit down between generations. However, because the marriage rules are supposed to be about not having inbred kids, these rules only apply to the heterosexual relationships, though like most social conventions, the social expectations are applied based on the observed gender of the individuals rather than impregnability. So gay relationships between the Days are expected. The other reason I choose this because the trans person hiding as the wrong gender is a fun reading of the Ki'O stories.

The male homosexual relationship is the Night, the female homosexual relationship is the Day, and the two heterosexual relationships are named Evening if the woman is Night and Morning if the woman is Day. This not the original/I'm centering the homosexual relationships by giving them the same words as the surnames.

I propose that the forbidden relationships are known as Dawn and Dusk. Dawn for if the man is Day, and Dusk for if the man is Night.

I imagine in a society like this, nonbinary people are outcast from the system, as they have no terms to describe themselves.

This, I think is because the sedoretu is a writing device in a story about binary het angst.