Perhaps that was the point; Reed loved it when Emerson let go and wasn't always so composed. Reed had managed to knock off some of the serious always proper sides of him, but it would seem it was still Emerson's natural reaction.
He continued to kiss him deeply, but changed the movement of his hand. He had his palm wrapped around the head, but his thumb on the very tip, kind of like it was a muggle videogame and he was using a joystick. He continued to rub circles with his thumb but moved his palm up and down over just the head of Emerson's erection, concentrating all the pleasure on that most sensitive area.