You are exactly right about marriage being foundational. But there is artificial insemination, cloning, or parthogenesis, or whatever type of replication in sci-fi.
Many wrote about distopian futures then. It’s not like whatever they dreamed up and wrote about had to work, or be benificial to society in order to get them paid. Sci-fi authors often write about societal changes that they or their readers would never sign off on in real life.
I think that it was something so alien that it could only come from actual life, and after the concept was broached in culture. I do remember societies in classic sci-fi where moral corruption had spread so far that folks were simpering all over the place, as in they were implied or actually stated to be homosexualists or worse. Like the main villain in Dune or in few of Anderson’s Ensign Flandry stories. This has happened in actual historical societies as well. But never “gay marriage.”
I actually tried to google the first mention of “gay marriage” in science fiction, nothing came up, on the first page anyhow. It’s like it came out of nowhere. As far as I can tell we are truly living a future that never was predicted in speculative fiction, at least as concerns “gay marriage.”
Freegards
Back in the ‘70s, feminists advocated the abolition of marriage claiming it was “sexist” and “patriarchal”. When that didn’t happen, I figure the left decided on another way to achieve their goals: make marriage so utterly devoid of meaning it would basically define it out of existence. Taking away the gender and numerical requirments for legal marriage is a way for them to accomplish their goal, i.e., replace the natural family with the all powerful state.