He didn't need a reason, mainly he got jealous and possessive because he had no reason to be, so indulging in those feelings was fun and not at all traumatic. Tony's smug smile just encouraged him really, because he didn't seem to have any objections.
Steve leaned down to kiss the back of his neck, lips dragging down his spine as he unfastened his trousers. "So you can watch," he said when he met his eyes again in the mirror. Steve placed Tony's hands on the sink, leaning him forward a little in the process. "And to give you something to hold onto," he added.