That Steve could drum his fingers against the arc reactor was just weirdly endearing to Tony, because Steve was just treating it like it was normal; a part of Tony as opposed to something that was fragile or to be avoided. There weren't a lot of people out there who would do something like that.
Tony was still pretty focused on taking his pants off though, even if Steve was slowing down, ready to have a conversation that yes Tony sort of wanted to have because he was curious. But also did not want to have, because it wasn't his business and it wasn't really all that nice of him to feel some amount of jealousy over Steve doing what he had to do while Tony was otherwise unavailable.
But, oh, wasn't permission grand? "How about you tell, so I don't seem like a terrible person?" He countered.