<pedant hat>
"What if" makes it (excuse me, not sci fi, but) SF. Some supposition, based in the possible or at least defensible, that is different from the here and now (though not necessarily both).
I know you guys were joking, but:
You don't need spaceships for it to be SF.
You don't need aliens for it to be SF (my published books have no aliens in them).
SF can be set in the past, present, or future, and it can be set right here on Earth in 4000 BC or 1944 or 2009, and still be SF.
It's the "what if," which has to be the essential element of the story. A romance novel set on a spaceship is still a romance novel; if there were no romance, there'd be no story, no matter how many spaceships and rayguns and alien planets there also are. My three books are still SF, even though they include a relationship; the future they're set in, fighting the Cold Minds, is the element without which there'd be no story. Iain and Linnea could just be best pals through all three books and there would still be a science-fictional story.
</pedant hat>
ETA: We're all aliens. Or so I've always believed. From near the end of my unpublished first novel:
. . . I remember what Kelru was first to teach me: that we are all aliens, one to another. All of us, men and women, people of Earth and people of Windhome. And when we meet, all we can offer each other is kindness. Or love, if that is granted to us.