I was actually thinking about all the sci-fi from the 40s-60s, those who speculated as a living for fiction. I can think of several places that polygamy comes back as acceptable (at least to the state and society at large), Heinlein’s “the Moon is a Harsh Mistress,” for example. I think that is more polyandry or joint marriage contracts even. Nobody predicted or even put forth as a concept, even in a bad light or whatever, “gay marriage.” I mean wild concepts like perfect sex change is there, cloning, third sexes, time travel to become your own ancestor, but nothing about “gay marriage,” at least that I have read. It is truly a case of the truth being stranger than fiction. And they even had homosexualists, usually portrayed in a very un-politically correct light.
Freegards
Probably because I figure back then even science fiction writers thought that marriage had something to do with the survival of the species.