Well... when I read the website profile of Mushroom Melvin it said that the f + b is carved into him by an employee, because he saw some old VHS tapes where it is heavily implied that Bonnie and Freddy had a thing... So it most probably is cannon in the story Fazbear Ent. gave to the public. Although I don't think the actual kids trapped inside the robots would be sexually attracted to each other. I hope that is not the case. So in short the fictional Bonnie and Freddy as characters by Fazber Ent. probably are cannon :(... But the animatronics themselves are not because they are just kids.