"I am a complicated suspicious bastard," he avows. "And I also happen to know that the first time Tony fucked you it was at least in part so you would not go home and murder him for a few more hours. I expect it would put a different cast on things if the other Sherlock were aware of that."

The reason, the crucial difference in this case, is obvious to him and has little to do with Tony. But he is not even going to think it very loudly. There are some things he simply cannot betray.