In the expanse group marriage is the norm on earth. The main character has like 8 parents who lead an idyllic life on a ranch in Montana.
polyamory
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BBC's Trigonometry was a very sweet story about a monogamous couple that slowly falls in love with their new roommate, and the challenges it introduces into their idea of relationships.
Gemma and Kieran, a cash-strapped couple in busy and pricey London, open their little apartment to a third person. Ray, their latest addition, somehow makes the flat appear larger, not smaller. With an extra pair of hands, many tasks grow easier, nicer, and better with time.
“The traitor baru cormorant” has a main character raised in a culture where polyamory is the norm. Great representation throughout the series, book 2(? It’s been a while) has solid nonbinary representation as well.