So Steve’s been trying to convince Tony -slowly, as much as he can without pushing too hard- to adopt kids.
Tony always exits stage left as fast as is humanly possible. Once, he even blows something up to distract Steve long enough to punch the repulsors and fly off.
Then, when they’re at a crappy gala which they all hate-
Natasha’s off to the side with Bruce, teaching him how not to go on a vicious homicidal spree when one of the people here tries to talk to you (it’s harder than it looks, seriously), and Tony has finished his quota of schmoozing for the night. He’s looking around, casting world-suffering looks at the other Avengers when he finally catches Steve, and he stops, frozen, hand outstretched to tap him on the shoulder. Because Steve is bouncing a baby carefully in his arms, with this small, intimate smile on his face, looking down at it like he’s holding the world bunched in baby booties and a diaper.
Tony hears him tell the mother that he’s beautiful, and the mother gushes before scooping the baby back up in her arms and spinning him, gliding back into the crowd.
Tony just stands there, shock-still, until Steve notices him.
Steve turns to him, bashful, like he’s been caught doing something wrong, and Tony knows that if he presses up against his chest, his arms, it’ll be achingly warm where the baby was.
Tony looks down at his feet. He swallows; says, “So, they seem less horrible than I originally thought.”
And steve’s answering smile makes him dread the dirty diapers, the crushing responsibility, and his own monumental daddy issues a bit less than he did a minute ago.