What do Gandalf and Magneto have in common? Both are played by Sir Ian McKellen. Both are figures of literature, one being a novel, the other being a comic book. And both have porn written about them. Slash fiction is a genre of stories written by fans about science fiction/fantasy characters in erotic situations. We know this because McKellen brought it up.
McKellen is like your dirty old grandpa, if your dirty old grandpa was British. He’ll make totally unexpected comments about pornography and sexuality.