What is science fiction?

For much of my reading career, I've been reading science fiction and fantasy.

I have noticed a trend among science fiction readers to discriminate against fantasy as if it is some sort of lesser fiction.

Well, it is quite ironic that science fiction readers would do this, considering that science fiction itself has been considered inferior for most, if not all of its existence, something that continues to happen now.

It brings up the question of "what is science fiction?". Most people know it when they see it, but having discussed this in detail with a group of friends, I realize that there is disagreement and no clear definition.

Is there anything that automatically makes a novel science fiction? I think there is... time travel, stories set in the future, stories with technology that doesn't exist yet. That all seems reasonable to me. Still, there are probably plenty of examples of these where a novel or story has these elements but aren't considered science fiction.

Some examples of "disputed science fiction" are: "A Handmaid's Tale", "The Time Traveler's Wife" and "Slaughterhouse Five". The only reason that they aren't considered science fiction is because their author says that they aren't, or they don't "feel" like science fiction. I think they all are.

It is possible for a story set in the future NOT to be science fiction? Is is possible for a story with time travel in it not to be science fiction? What about aliens? What about new, future technology?

I'm tempted to say "No, it's not possible.", but I'm willing to listen to counter arguments.