It seems like every time this comes up, I end up arguing with someone that Star Wars is INDEED properly called Science Fiction.
Not that you CAN'T refer to it as "science fantasy" or "space opera" or "space fantasy" or some other fancy name, but I don't think you can properly deny that it is also Sci Fi, unless you mangle the definition of Sci Fi such that it no longer applies (ie: that most Sci Fi then becomes "space fantasy" or "science fantasy" instead).
Often they'll cite that Lucas used Joseph Campbell, or that it has some similarities with Lord of the Rings, but typically they buy into the hype generated by Lucas since the success of the first two films, and stuff he's said recently, as pretty much the final word.
My pet theory is that this is some kind of defense mechanism used by certain SW fans to try to make it so you can't compare it with other Sci Fi (like Trek), or to set it apart to make it seem "more special" and Lucas seem more visionary (like "Sci Fi" is "too nerdy" or "too boring" a label to apply to Star Wars).
I try to point out the high "fantasy" content in other acknowledged Sci Fi like Trek, Star Gate, Doctor Who and so forth (ie: ghosts, gods, demons, magic, destiny/fate, religion, elements from the "heroic journey," parallels with ancient mythology and holy books, etc).
Here's some comments I got from someone while discussing a fan edit of Star Wars, which are pretty typical for what I often hear:
Note: this began as a discussion of turbolasers (which I explained were not lasers, despite the name).Wow. So you're not familiar with the idea that Star Wars is a "space fantasy?" In a fantasy, you can get away with whatever your imagination can cook up. So what if a Turbolaser is not like a real laser. No one cares about that anymore than they care about sound in the space scenes, or that Chewbacca doesn't wear pants.. Just enjoy the movies for fun of it.
It's not an excuse by the fans, it's the truth. Star Wars is a "space fantasy."
It's mentioned in the documentary, "Empire of Dreams," that Star Wars IS more Space Fantasy than Science fiction. And the more recent documentary, "The Legacy Revealed" goes into more details that prove my point.
"Science fiction consists of improbable possibilities, fantasy of plausible impossibilities." -- Miriam Allen deFord
Not all Sci-Fi takes place out-side of Earth or mentions space travel.
"Back to the Future" is sci-fi (with a blend of comedy, but sci-fi for the most part) and it setting stay on Earth, and involves time travel instead of space travel.