That soothing voice wasn't something anyone else had heard, even in jest, but especially in sincerity. This softness in Jim was reserved for Matthias and Matthias alone, because Matthias was the only person in the world that Jim had ever (possibly would ever) trusted with it. Even if it was never spoken, Jim was sure Matthias was enough in tune with him to know that, deep down. It was likely something Jim would never say out loud, for obvious reasons. He couldn't afford that kind of luxury.
Not that he was incapable of these kinds of attachments, clearly he wasn't, if the entire saga of his and Matthias's courtship had anything to say about it. Jim wasn't incapable, he simply wasn't practiced in it, and therefore it was still largely outside of his comfort zone. Even Moriarty had not been immune to the deeper relationships in life, however twisted they were, and however twisted this relationship with Matthias was still. It was far from innocent, and they both knew it, but neither of them seemed to mind. Moriarty and Jim were too paranoid for any relationship, twisted or otherwise, to last, but Matthias was different. Jim wasn't too proud to admit that he'd rather keep this one, if he could, so there was that. He'd already made his claim on Matthias.
Jim kept one hand on his hip, letting the other begin to idly trace the scars on his back again with a finger, watching Matthias undress with an intensity that would probably bore a hole into the man's skin if Jim were capable of that. He wasn't, he was still just a man, the only power he had was in manipulating other people and orchestrating elaborate crimes. Well, there was one other thing he thought he was particularly skilled at, something that would actually be appropriate for this moment. Before Matthias could push his slacks down all the way, Jim grabbed him with his free hand gently, but firmly by the wrist to stop him.
"Not yet." His voice was still soothing, not at all menacing, but he meant it. He wanted Matthias to give up his control another way. The hand on his hip moved to snake his whole arm around Matthias's waist, breath hot on the skin of his neck as Jim mouthed at it, teasing him in the trap he'd just set. "Let's give you something to remember, then."