"Swear it." John whispers into Sherlock's ear. "You tell no one." It's not really about John's reputation. He doesn't have one, not really. Especially not here. But he doesn't feel it's anyone else's business what he does with Sherlock. And in a strange way, John's afraid that he'd get some sort of competition. Right now, he's feeling horribly greedy. Sherlock is his. He doesn't want to share him, not like this.
He wants this to be their secret. And he's okay with this happening between them.
He moves to kiss the front of Sherlock's throat, gently nipping at it as he moves towards his jaw.