Sherlock has never been in a long-term relationship before John, and is worried that John will think their sex life is boring. As the pressure grows in his own mind, he proposes that they invite another participant into their bedroom. John is a bit shocked at the suggestion, but reluctantly agrees because he's now worried that Sherlock is growing bored him.

Cue the night it finally happens (Moriarty if possible, but any other guy aside from Mycroft would work just fine, even an OMC), everything goes smoothly... Until Sherlock and the other participant begin spit-roasting John. After several minutes the other guy wants to take a turn with John's ass, so Sherlock lounges on the bed and watches as the third party rolls John over and fucks him face-to-face - they end up kissing and basically making love while Sherlock inadvertently becomes the third wheel. Sherlock is not at all pleased, but doesn't know how to express this to John the next day as the entire thing was his idea.

Bonus Points if John isn't some sex god who's super comfortable being spit-roasted. In fact, he's never done it before and is kind of desperately uncomfortable at first.