You weren't wrong to start a fanfic in the first place! You were just having fun and using your imagination, which is not wrong in any way. And writing a love story between them or smut or whatever, is indeed fiction. The definition of fiction is, literature in the form of prose, especially short stories and novels, that describes imaginary events and people. In this case it's imaginary events. IMAGINARY. So that stuff is fiction, because it's never happened, nor will it ever. Maybe it will, and that would be awesome, but it most likely won't. You weren't doing anything wrong. But I, guess it's kinda too late.(?)
Don't be sorry. It's fine.