Billy's answer had Tony wondering who's room was christened by the baby-Avengers with a teasing smirk, then how different the mansion might have been in Billy's universe. The team Tony knew seemed to be pretty familiar to the out-worlders, so he could safely hope he, or someone well deserving, played a part in the inspiration. Nick's blowjob experience was more than enough to derail that thought train, though, and Tony gave him a disbelieving stare for thinking Billy really needed to reexperience that tidbit next time he was getting head in a dark theatre.
Since Tony wasn't as enthusiastic as Nick for the unprompted overshare and no one else was piping up, he didn't have to explain why the tidbit led him quite naturally to take the next question. "Most embarrassing fantasy," was his prompt, and he looked pointedly at Billy as he did, like he knew the kid had some seriously lurid masturbation fodder lurking in his imagination.