Sherlock had deliberately spread his legs out a bit to make John have to work harder to get his pants down, but eventually pushed them back together after letting John do the hardest part, and let gravity once again take over.

He pat John's head, like one would pat a dog who brought his master his slippers.

"Good boy." He said with a slight smile.

"Now only one more task before I give you the privilege of sucking me." He said, the praise in the voice gone.

He turned around, presenting his arse. "Clean." He said."You can use your hands to open my cheeks, but nothing more. A good cleaning, then you can have your reward."

As with everything fastidious about him, he had made sure to clean this part of himself well. Of course there would always be his musky scent, even after a wash, but all of him was totally clean, there was no chance of.. contamination.