That was an invitation. Watson knew that, and he wouldn't have done such a thing if things were in any way otherwise, but it was an invitation, and he was in absolutely no state to do anything other than accept it.

The grip of his hands on Holmes's hair softened a little, his fingers coming to rest at the back of his neck, and he thrust into Holmes's mouth -- slowly at first, biting his lip to hold himself back. This beloved, aggravating, beautiful man, who was perfect, and who would let Watson have this liberty. If he were going to fuck Holmes's mouth, he would take the utmost care about it.

He increased his pace a little, gasping at the air, still gentle.