Believe it or not my experience is quite similar to yours. In junior high school my best friend and I used to write porn to amuse each other. We created our own characters and took turns writing chapters to see who could be the most... uh, creative.
We were both in an advance writing course and this was our own special type of extra curricular activity.
I didn't discover fandom and start reading/writing that until about 20 years later. It started with Buffy the Vampire Slayer. I'd pair Buffy and Spike, then I realized I liked Spike with Xander or Giles more than Buffy.
Harry Potter was a quick skip from there. And now its Harry and Draco forever.