"It won't hurt."
This is a lie. It's going to hurt, but Tony covers the lie with a smooth grin and a comforting pat to Steve's firm bicep.
Steve's not dumb, and he's been sleeping with Tony long enough to know when something isn't right, a time frame measured in minutes for most of Tony's partners. He raises his eyebrows, keeping his mouth in a thin line.
"I do this all the time."
Steve frowns. "Why?"
"It feels good. Let me show you." It's the right amount of sweet and sassy, with a veneer of care. Every person Tony's ever slept with has required a particular level of handling. Steve's pretty, but he's not any different from Rhodey the first time Tony built up his ego before swallowing his dick.
Without trust but with an adorably patient sigh, Steve rolls over onto his back. He doesn't have a spare ounce of fat anywhere on his scientifically-sculpted body, a fact which Tony takes a long moment to appreciate with a careless nip at the skin of his hipbone. He doesn't like considering that his father essentially helped construct an Adonis of a fuck buddy for Tony, so instead he gives Steve's cock a friendly lick. Warm blood starts to fill him.
"Ah, not yet. We need to get you nice and slick, and not too hard."
He squirts a large gob of lube into his palm, and passes the pump bottle over. He warms the slick yet viscous goo in his hands before slicking his forefinger and playing with the pucker of Steve's ass. Steve squirms, even as he warms his own lube and begins stroking himself. He masturbates like a Catholic virgin, ashamed of touching himself and excited by how naughty he feels every time. Tony bends over to blow a cool breath against the wet surface he's making, and is rewarded by another squirm.
Tony delves inside, delighted and aroused by the heat clenching his finger. He's patient now, stroking and sliding, ensuring Steve's ass is stretched and slick. Yes, this will hurt, but it doesn't have to make him tear, or bleed.
The moan he lets out as he starts stroking faster is Tony's cue. "Stop now." For a moment, he thinks Steve will just keep going, jerk himself off, and lay there in an orgasmic puddle as Tony fucks him. Tony can't say he isn't intrigued, but they've done that before, and he's taken Steve's generous dick up his own ass.
It's time Steve gets that pleasure.
"I'll help but you need to do it. I don't want to bend you in half by accident." He's done that before. His own dick loses some of its interest just at the memory of pain. Tony shoves that away.
He takes hold of Steve's hand, and helps him guide himself. Tilting and bending is an art, one Tony has learned with trial and error. Not too hard, not too relaxed. Steve gasps as he feels the length of his own cock pressing backwards. "I can't. I can't. Tony, don't."
"You can." He won't push him now, not yet. "It feels like it's going to hurt, but it won't."
He takes Steve's cock from Steve's nervous hand, and with a careful eye, presses just the tip against Steve's own hole. "Now push it in. You can do it."
Steve fumbles around Tony's hands, and with his face drawn up in anticipation of pain, he uses the tips of his fingers to push the head of his dick inside himself. His body shudders. Tony's mouth goes dry from the sight. "Just like that," he manages to say.
Steve's words have left him, just incoherency mixed with a soft whine. If Tony lets him stop now, he'll never get inside. Tony strokes him down at the base, tickling his fingers up the shaft and pushing him deeper inside himself before he strokes back down again. "You do it," he says, putting Steve's hand in the position to stroke himself in and out with the inch or so of give he's got.
There's an "Ah!" and an "Ow!" blended as he clumsily pushes deeper and sets up a rhythm. Tony sits back on his heels for a moment to admire the perfect view: Captain America is supine, pornographically debauched, fucking himself with his own cock. Tony really hopes Jarvis is getting good pictures of this. He wants to take these out again someday.
"How does it feel?"
"So good," is the immediate reply. Steve's eyes are squinted shut, his mouth open. Tony thinks this is probably the first time he's ever fucked or been fucked without a condom, and he feels like he's having a threesome. "I'm going to...." He breaks off. Steve hates saying dirty words, sex words. He blushes when he says 'prick' and even when he wants to suck Tony's dick, he can't say so.
Tony says it for him. "Come. Cream yourself, Steve. Fill your tight hole with your come." It's corny, but as much as Steve can't speak nasty words, he comes like a fucking racehorse when he hears them, a series of groans shooting out from him as he jerks and spasms on the bed, filling himself.
When his dick falls free, and his ass is leaking white semen, the visual is as amazingly hot as Tony thought it would be. He's been hard, hard for hours, for decades, just watching this, and he doesn't know how he rolls the condom on without fucking it up before he gets to the fucking. The slide and squelch of fucking another man's semen into Steve drives him on as much as the quakes still rocking Steve's body, and the tight fist of his ass clenching down just the way Tony likes. His head rolls to one side, a little lazy now that he's come, and Tony almost smacks him upside the head to pay attention.
"Nope, just enjoying." The bastard actually puts his actual hands behind his head as Tony hoists his legs up for a better angle. Just for that, Tony pounds in harder, and enjoys the sudden wince.
"Next time," he huffs, "we'll shove your dick in there and mine too. Fuck you wide with two."
Steve's eyes fix him with a wary look, unsure whether to believe him. "Wouldn't fit," he says.
"It would. And if you like it, really like it, we'll ask Bruce to join us. I can insult his mom while we're both buried inside you."
Steve's wariness turns to outright fear. Tony grins, and there he is, riding the slick come and the look of worry all the way to his own orgasm.
Tony knows himself, and he knows exactly what he needs to get off.