There is no such thing as a "science-fiction" genre. Or, for that matter, a fantasy one. These are just labels applied to books with certain narrative elements by people who are deeply uncomfortable with made-up things, and want to be sure that they can avoid any work of fiction that they can't pretend is real.
The proof is this: You can find a "pure" horror story, one that crosses no genre boundaries and is plain and simple an attempt to scare the ever-loving crap out of someone. (Like, say, "Friday the 13th", or "The Collector".) You can find a "pure" comedy, a "pure" romance, a "pure" drama, a "pure" adventure...
But it's impossible to find a science-fiction story that isn't also part of another genre. "Star Wars" is sci-fi/adventure, "Alien" is sci-fi/horror, "Ghostbusters" is sci-fi/comedy, "Dark City" is sci-fi/mystery/horror/adventure/horror/back-to-the-mystery-for-a-bit/romance/adventure...it's no coincidence that science-fiction/fantasy tends to be a pioneer in genre-blending. How can it not? It has to steal other genres simply to exist.
"Science-fiction" is simply a collection of narrative elements, involving speculations based on what we know of science, applied to stories in other genres. "Fantasy" is similar, but involves speculations based on what we used to think was true in science but now know better. (Which means that Isaac Asimov's "Lucky Starr" books went from being a sci-fi adventure series to a fantasy adventure series over the course of sixty years.) Neither one is a genre.
I am, of course, open to contradictions on this. Anyone want to cite a "pure" sci-fi or fantasy story? (Keeping in mind, of course, that "satire" is a genre.)