sherlock is wearing his pajama pants, watching tv or something equally mundane, and for some reason, gets an erection.
because he's wearing such thin, loose trousers, it makes the "tent" really obvious. sherlock tries to hide it, but john notices anyways, and helps him take of it (established relationship is preferred!)
basically, i'd like to see descriptions of sherlock's clothed hardness and possible pre-come stains soaking through his pajama bottoms. ... thank god for anon.