Steve hates hearing Tony swear. He frowns and chastise (‘it’s impolite, Stark’) Tony, only to get more cursing and perhaps some laughter for his efforts.
Steve starts to slip, stops reprimanding Tony because he is too busy watching the play of coding and schematics and light across the genius’ face.
Steve knows he is in trouble when he starts to find it endearing, starts hiding laughter when Tony curses a blue streak halfway under a car, tools clattering to the ground.
Steve starts to love it when it’s pressed into his skin, slurred between ‘please’ and his name and ‘more’.