Mycroft has a magic cock, and is shameless about using it to get what he wants. (Anyone he fucks can literally never refuse him anything? He's so amazing in bed that they spill out their darkest secrets and he rose quickly through the ranks of MI6? He gets his bed partners to show their freakiest kinks and then blackmails them?)

In any case, Mycroft is a fantastic fuck and Sherlock is more than a bit grossed out by the whole thing. Especially when he finds out that {Lestrade / Mrs. Hudson / Molly / John / hell, even Harry} are among Mycroft's "conquests."