There's really no good reason why "fantasy" and "science fiction" are so frequently spoken in the same breath.
One of them usually involves machines, the other usually involves magic.* Strangely they both seem to hit upon time travel on a regular basis. But personally, I see a pretty clear division there.
Yet go to your local bookstore. If fantasy and sci-fi aren't regarded as one section, they are certainly shoulder-to-shoulder . . . or spine to spine, I guess.
I wish the bookstore would just be honest, put a big sign that says "NERD" over the whole thing and be done with it.
It's too bad that those genre's are labeled, and at times dismissed, as geek territory, because there's also no good reason why "things that are or could reasonably be real" make for better or more legitimate storytelling.
In fact, the conceptual free reign that "magic" and "future technology" give you as an author is a powerful tool. "Back to the Future" (here I'm discussing the first movie, not the trilogy) purports to be about an awesome car that travels through time . . . and that's what it is indeed about, to some extent. But it's also a discussion of generational gaps, and how the path to adulthood involves reconciling your parents as real people, not just as the images your young mind constructed for them.
Harry Potter treads this ground, too. The narrative difficulties of character study are easily overcome when your setting includes bowls of people's thoughts.
In most stories you have to really put your character through the wringer to draw out his nature. In fantasy? Nah, bowl of thoughts! Done.
*And please don't trot out the old line about technology being magic. There's a difference, you know it. Close your Macbook, and walk away.