Wet Christmas (http://www.livejournal.com/users/donnaimmaculata/599.html#cutid1) by Donna Immaculata. (From my recs page: The main character, Bill, was not born with the knowledge and experience of a sex god -- he is young and inexperienced and learning as he goes. Wet Christmas takes place in the shower, with Bill doing a little self exploration... with Scabbers the rat watching on. Wonderful descriptions and characterizations as well as totally believable internal dialog makes this a very enjoyable story to read.)

Is that slashy enough?