Half a breath before Steve made his move, Tony vividly realized how open he was to attack and only had the time to choke on his air and tense. His ticklish sensitivity should have been reserved for the gentle attention of a lover, and only the blackest of villains should think to take advantage of such a sweet thing. Tony went 'Ha-ah-nnn!' and did collapse, tucking his arms to his sides and curling in to protect as much surface area as possible from the cruel assault, and sputtered to contain the giggles he would not allow Steve the satisfaction of eliciting. He would not.
While he made a weak bid to fight back, the best Tony could do was scuffle backwards and try to wrench free of Steve's clutches. But if Steve got away, then he might avoid due retaliation when Tony had recovered his senses, so, in direct conflict to the escape tactic, Tony kept a fist balled in the front of Steve's shirt to drag the man with him. Eyes squeezed shut and stumbling blindly, Tony was stopped short by the desk behind him, knocking it a few horrible, screeching centimeters across the floor and bruising his ass all over again. He gasped, "Fff-uck,"-- realized he was giggling, and maybe could have screamed-- and slumped with his weight against Steve's shoulder and hand still tangled in his shirt while the pens and paints clattered and rolled to a stop in the desk drawers.