This one’s for persian-slipper:

Sherlock is on record as not caring for sex.  He doesn’t like the creepy intimacy of one person inhabiting another person’s body, or the emotionally flaying vulnerability of being driven to uncontrolled pleasure at someone else’s hand.

He knows John is sad about this, because John likes sex, and intimacy, and the wild panting risk of being driven to uncontrolled pleasure at someone else’s hand.  So when John comes home one day with a large and gracefully-shaped dildo, and a strap-on harness to hold it, Sherlock is willing to give it a try, just for him.

It turns out that when he can keep his faculties, driving John to clawing, thrashing pleasure via a protracted bout of intercourse is fairly bloody fantastic.