“If we lived in the future and we had teleporters and I wrote a story which had teleporters in it, would it still be science fiction?”
Maybe. Imagine you wrote your story as a 1960s story about teleporters. If you did it would still be like a science fiction story from that time and so it would still be a science fiction story in general.
It is not whether the technology exists or not that matters but whether you treat the technology as fictional.
Think about it. Dinosaurs existed but if you wrote a story about a dinosaur like me trying to make her way in the popular song writing industry of 1960s New York, your story would be science fictional EVEN THOUGH I REALLY DID THAT! Do you see?
OK, an easier one. Aliens visit Earth in secret – science fiction right? But what if it turns out aliens really have visited Earth in secret? Does any SF book dealing with that issue just become contemporary fiction?
Fiction isn’t fiction because it is false but because it does not HAVE to be true.