I think the sexual orientation aspect of Sherlock is inevitable given Doyle's original text, which does construct a very close relationship between Sherlock and John, and which also explicitly presents Sherlock as not being into women. Certainly there are a lot of Sherlock versions pre Mofftis's one which play up the homoerotic subtext - and I think there is definitely a homoerotic subtext. The fan fiction focus is partly the tendency of fan fiction generally to focus on romantic relationships, but it also responds to the show's shameless and frequently frustrating games with homoerotic tension.

If you want Avengers fanfic in the John Steed/Emma Peel sense, see http://archiveofourown.org/tags/The%20Avengers%20(TV)/works, but you'll have to weed a bit as some fic writers apparently don't have much truck with tag coherence, and there's some bleed into Marvel Avengers owing to mislabelling.