The Virgin and the Camera Phone
I love how when Sherlock’s emotional maturity and sexual experience is in question he doesn’t defend himself overtly to Irene (“Have you ever had anyone?”), or Mycroft (“It’s to do with sex.” “Sex doesn’t alarm me.” “How would you know?”), or Moriarty who, according to Irene, calls Sherlock “The Virgin.”
At the moment of truth, when Sherlock dramatically reveals Irene’s password, it’s John’s opinion of his heart that nags at him. He goes so far as to articulate his own understanding of love in opposition to John’s. This proves that Sherlock has contemplated John’s thoughts and feelings throughout the episode, without the two ever having discussed them directly. John is his conductor of light. Without his influence Sherlock wouldn’t have deduced the password.