Eye of the Beholder

At a crime scene, John confesses that he's in love with a man. Sherlock is upset until John starts describing this man in more detail. For just a few seconds, Sherlock thinks it could be him... until John mentions that this man is also beautiful.

Sherlock makes a cutting remark and walks away.

John is devestated... until he works out what's really happened and sets about proving to Sherlock that he is beautiful with long, drawn out, worshipful sex.