Sirius dresses up as Father Christmas for some reason (they're doing a Muggle Christmas at Hogwarts or the Burrow or something). Snape is there.

Unfortunately (or fortunately, depending on how you look at it), Snape has a Santa fetish. Perhaps he has a thing for beards, fat guys, or red suits. Or perhaps he used to masturbate on Christmas Eve, to keep himself awake as he waited up for presents, and this warped his young mind so that now he gets aroused when he thinks of Santa.

Hope this is kinky and potentially humorous enough for you. *g*