I'm looking for platonic sex/sexual situation fics. By this, I mean fics in which they engage in some sort of sexual activity (because of an experiment, maybe, or one of them needs sexual assistance) but just as "friends who are helping each other out." A few I can think of that fit the bill: aghast's "Come As You Are," in which Sherlock has never had an orgasm and John jerks him off so he can experience the sensation; rubyofkukundu's "Keep Calm and Carry On," in which Sherlock gives John a handjob in order to test an alibi; and CumberbatchCritter's "Dysfunction Junction," in which Sherlock takes viagra and John tells him how to satisfactorily get himself off, listens as he does so, but doesn't participate.
Any suggestions? Watching porn together, wanking in front of each other, giving very frank sexual advice, etc., are all great. It's also fine if the platonic sex turns not-so-platonic in later chapters.