John was straight before, but now he's a bottom for Sherlock, in an established relationship, because he loves him. They make love lying on their sides in bed, slowly, with all the lights off, and sweet words are whispered. Sherlock loves John, and that's why he never asked for more. But John doesn't know that Sherlock used to be a Dom before, one with a reputation and a very dirty mouth. Things change when Victor, old friend and once Sherlock's Sub, unexpectedly comes to London and visits. If he knows about John at all, he believes their relationship to be as open as all of Sherlock's used to be. When he falls to his knees in 221B and begs Sherlock to let him suck his cock, Sherlock can't resist. It may happen again occasionally, or only once, and Sherlock hides it from John while trying to justify it to himself. John eventually finds out, and he's hurt at first, of course, but I'd love to see John determined to prove to Sherlock that he can learn this too, and be up to Sherlock's high standards, just as well.