Very well written - excellent use of present tense and first-person - and Lupin and Sirius were wonderfully characterised. I'm not much of a fan of ship fics, so although it isn't a flaw in your fic, I'd love to read more Lupin and/or Sirius stuff of yours that focus on the characters instead.
The only real criticism I can think of is that I dislike the use of the word "cock" in what is otherwise a wonderfully subtle and classy piece of writing - maybe implying the deed or using a euphemism would be more effective.