Fantasy and science fiction are usually lumped together under the "speculative fiction" umbrella. Since Clarke pointed out that magic is equivalent to sufficiently advanced technology, the dividing line between science fiction and fantasy can change according to the technical sophistication of the observer.

In 1865, Jules Verne's _De la Terre à la Lune_ was science fiction. By 1903, Tsiolkovsky made it space fantasy. By 1960, it was back to science fiction again, though it will never be usable for manned launches until we find some way to make >100g accelerations survivable.

As such, I don't find it useful to divide the genres too strictly. A Charles deLint fairy tale novel and a Cunningham/Carruthers Men in Black comic are in the same genre: low modern urban fantasy. Laser guns and magic wands are the same thing. Lightsabers and vorpal blades are the same thing. Robotic automata and servitor daemons are the same thing.

In most cases, the fantasy elements keep the story interesting, and the sci-fi elements lend it an air of future plausibility. You don't have to explain a magic wand, but you might have to mention that a laser rifle uses an Er:YAG laser and a heavy air-breathing lithium battery, only in very arid atmospheric conditions, to literally explode the water in a human target into steam from hundreds of meters away. But a shock cannon uses a different type of laser to ionize the air between the user and the target, and uses a voltage multiplier circuit to shoot an artificial lightning bolt down the conductive pathway, grounding through the target. If it isn't explained, it's another magic wand.

I have read books that started out looking like high fantasy at the beginning, until the characters learn that magic is the result of an AI--from the colonization ship's navigation computer--performing weapons research on the human descendants of its former passengers, and that the creatures formerly described as demons are actually aliens.

Try to pigeonhole that one into the right genre.