Heee, fantastic, "Mystery Artist!" Everything about this piece, from their poses to the background details, is incredibly erotic. I love the fact that Severus' toes are curling and he seems so focused... It makes me sure that's he's either coming, about to come, or trying really hard not to come yet! Meanwhile, Minerva is just sort of taking her time and enjoying herself.
As I think a few other people have already mentioned, the background -- the colors, textures, and patterns -- really adds so much to the atmosphere, putting one in mind of all sorts of naughty bits, even without the action taking place. Great work. :)