The Avengers move in to the Tower and if Tony already had a mild obsession with Steve’s perfect little butt, it was just getting worse, having it real close and walking right next to him.
He keeps dropping things so Steve, being the great person he is, helps him pick them up, he asks for help to get stuff from under the couch, reach stuff in the high cabinets…
One day, after a training session he moves forward to give a little slap to the left buttcheek.
After that day, and Tony explaining that “is a friendly gesture this days! good job buddy! you sure had that in the 40’s!” and finds any opportunity to get his hands on that butt, Steve finds himself all flustered and blushing (“I don’t blush!”) and a little worried everytime Tony is near.
Tony hears Clint explaining to Steve after the Captain tried to slap the Hawk “I mean he surely doesn’t touch my butt! i wouldn’t mind tough!” and laughs.
So then Tony starts slapping the whole team’s butt, to not look suspicious, or so he tells himself, until he grabs Natasha and ends up with a swollen eye and his head smashed on the floor.
Steve helps him up, puts an ice pack on his eye, gives him painkillers and leans in to kiss his forehead.
Then Steve is kissing everyone’s forehead to also not look suspicious, or so he tells himself. “You’re both idiots” Clint says, Bruce blushes and Thor returns the kisses, Natasha giggles, and gives his butt a little squeeze.
They later have to deal with Clint squeezing everyone’s butts and laughing like a maniac (“How come he doesn’t gets his head smashed by Natasha!!?”) and the kissing monster that is Thor.
Until the day Tony snaps, runs to Steve and hugs him fiercely from behind “This butt is mine, MINE MINE! HANDS OFF BARTON!”
The butt torture ends, Thor still kisses everyone, Bruce keeps blushing, Natasha giggles and Clint is happy.
Steve and Tony are too engrossed with each other to notice anything.