SciFi - Night people and day people

Anyone remember a short where marriages are comprised of four partners. Two men and two women. One of each from the day 'race' and night 'race'. Each spouse was intimate with their 'race' counterparts but they kept the one of their same 'race' as a platonic partner.

I don't think it's your book as the races aren't night and day people - but there's a similar theme in Fire Dancer by Ann Maxwell.

It's an Ursula K. Le Guin story, and it could be one of several she wrote in that world. Strange Horizon has a bit of write-up that could get you started:

Le Guin, making her third appearance in as many anthologies, offers a fuller and more satisfying exploration of the polygamous marital customs of the planet O (which first appeared in "Another Story, or A Fisherman of the Inland Sea" in A Fisherman of the Inland Sea, 1994). Ki'O, the people of O, traditionally make four-way marriages, called sedoretu, in which two women and two men engage in a cycle of four partnerships, two heterosexual and two homosexual. Bisexuality is the norm, and polyamorous sex is considered a natural part of family life, which, on the face of it, sounds rather civilized and desirously open-minded. But nothing is simple, and "Mountain Ways" (from the Tiptree Award-winning The Birthday of the World, 2002) is a cautionary tale, in Le Guin's practiced anthropological mode, that seeks to uncover the difficulties inherent in any attempt to systemize love or sexual relationships. Akal and Shahes, a lesbian couple eager to commit to one another but unable to do so without two men to make a sedoretu, perpetrate a (trans)gender fraud with potentially murderous results—clearly Ki'O society finds monogamous and committed same-sex couples as difficult to contemplate as does our own.

And here's an interview with Le Guin where she names some specific story titles.

ETA: Oooh, now I found story excerpts! From "Unchosen Love," in The Birthday of the World:

A moiety is half a population. We call our two halves the Morning and the Evening. If your mother's a Morning woman, you're a Morning person; and all Morning people are in certain respects your brother or sister. You have sex, marry, have children only with Evening people.

When I explained our concept of incest to a fellow student on Hain, she said, shocked, "But that means you can't have sex with half the population!" And I in turn said, shocked, "Do you want sex with half the population?"

I second Myriadbooks' suggestion.

Sounds really complicated, no?

Not 100% sure that's it but I'll go check it out either way. Thank you!!!

"Mountain Ways" (Ursula K. Le Guin) is probably it -- this story got quite a bit of attention on release.