I'm looking for a story I read a while ago where Kurt is at Dalton and rooming with Blaine. All is fine for the first week or so, but then Blaine starts masturbating at night when he thinks Kurt is asleep, but Kurt hears every second of it. This happens every night, and Kurt gets more and more sexually frustrated, until Blaine walks in on him masturbating one day. Kurt is horrified, but later that night he sees Blaine touching himself while watching Kurt and then sex ensues.
Sorry for the not great summary, but hopefully one of you lovely folks will be able to recognise this fic :)