To avoid further derailing the Worm thread:
I did say "a good chunk", not all of them; "fairy tales", "Norse myth" and "Greco-Roman myth" were all in the list of exceptions, as were quite a few classic works of literature like Dracula and Pride and Prejudice. They were still heavily outnumbered even in literature, though. And the main gay ship in the Arthurian fandom on AO3 seems to be Arthur/Merlin.