Every canon character was once an OC. OC x OC pairings are fine, aren't they? So what's the big deal over OC x Canon pairings? Yes, sometimes they can be annoying, especially in this fandom... But that doesn't make them wrong.
If a person pairs their Sonic FC with Tails, then so what? If a person pairs their Prototype FC with Alex, then what's the big deal? If a person pairs their Mass Effect FC with Legion, then why are Du guys ranting on about it?
They don't Liebe that person in the show/movie/game/comic/book!
That's not CANON!
It's a Mary-Sue!
They're not your characters; it just shouldn't be done!...