[Kye]

There is a movement for polyamorous marriage. A city in Massachusetts just recognized that arrangement for domestic partnerships. I think, like with same-sex marriage, most people would struggle to think of a reason to be against it that isn't rooted in appeals to tradition.

https://www.lgbtqnation.com/2020/07/massachusetts-city-begin...

For animals, there's the Harkness Test.

[jcranmer]

Note that the dissent in Obergefell (the case ruling that states had to recognize gay marriage) actually explicitly said "It is striking how much of the majority’s reasoning would apply with equal force to the claim of a fundamental right to plural marriage."