For me, SF is the marked category. It's the subset of fantasy that is concerned with technology and the advance of human knowledge, just as horror is the subset that deals with eschatology and the fear of death. SF is also (as you know, swan_tower-Bob) the subset of this ghettoized kinship group of books that has the most cultural capital, being the "masculine" side of the binary (with all those "masculine" binary traits) of which fantasy is the "feminine" side. Therefore, I think there's a tendency to assign it primacy/default status and to insist that it's "more realistic." (Because of course "realistic" is "better.")

But that's a different rant, and this soapbox hurts my feet.