Steve was wearing a Stark Industries t shirt. Tony stopped and stared. He dropped his mug, and only the fact that Barton was sitting right there saved the floor from a splash of cold coffee and ceramic shards.

"You okay, Stark? You look like you've seen a ghost!"

"Hmm? Oh, fine. Yeah. Fine. Just tired."

"Here." Barton pressed the mug back into his hand. "You obviously need caffeine."

"Yeah." So Tony went and rinsed and refilled his mug, carefully not looking at Steve. Then he ran back to his lab to try and figure out what was going on in his head.

The possessiveness surprised him. The way he wanted to mark Steve, pin him down, keep him safe. He'd always been the world's original commitment-phobe. It didn't make sense that he'd want to own Steve so badly. It had never come up before, and they'd been together for a few months now. They were at the toothbrush/drawer stage.

On the other hand, Tony thought as he sipped his drink, maybe it shouldn't have been a surprise. Maybe he should have expected this need to own. He'd always had that side to him, especially when it came to his toys and his cars. And people coming into his home. And his inventions. And he could go on. And he'd always had people who he considered his.

Mine.

Pepper and Rhodey, and more recently Bruce and Steve. Barton and Natasha too. Not Thor because he'd been away on Asgard a lot, and Tony didn't really know him as well as the others. So yeah, he had people he thought of as his. Maybe that did make him possessive. He just wanted to take care of them, love them, give them everything they didn't ask for. People who loved him for something other than his money or his brain were few and far between, so when they showed up, he wanted to keep them forever.

Getting maudlin, this started off being about how hot Cap looked in the t shirt, stop getting sidetracked by manly angst. Tony put the mug down on the table and leaned back in his chair. Okay. So Cap wearing Stark Industries clothes. With a big bite mark on his neck, so everybody knows he's mine. So no one else will dare touch him.

And yeah, okay, that was hot. Tony palmed himself through his pants slowly, considering. Did Steve know he felt like this? How could he? I've only just figured it out! Did he wear the shirt on purpose? Was he trying to get Tony to step up and claim him?

"Sir, Captain Rogers is requesting entrance." Tony took his hand out of his lap, and tried to look like he wasn't really seriously horny.

"Sure, JARVIS, let him in."

"Tony, you alright? Clint said you were acting strangely." And there he was. Still wearing the Stark Industries t shirt. He had Tony's name on him, stretched tight across his chest. "Tony?"

"Hmm, what?" Tony cleared his throat. "Sorry. Did you say something?"

"Are you alright?"

"I'm fine, are you alright? I mean, aren't you hot?"

"Hot?"

There was a dampness around the collar of the shirt, and in Steve's hair as though he'd been sweating.

"You're..." Tony pointed, and Steve looked down at himself.

"Oh, I was sparring with Natasha earlier. I took a shower afterwards, the water's from that." He smiled sheepishly. Steve had a vendetta against hairdryers, and Tony wasn't sure exactly why. "I grabbed one of your shirts by mistake, sorry about that."

Not only a shirt with Tony's name on it, but one of Tony's shirts. That made it even better, and Tony couldn't say why. His hand found its way to his lap again.

"And so you should be. Take it off at once." Tony gestured regally. Steve looked confused, then exasperated.

"Is that what this is? You're just horny?"

"Captain America said horny!"

"Well you are a horn dog, Tony. If that's all that's going on, I'll leave you to it." And he turned as if he was just going to walk out. Who does that?!

"Wait!" and yeah, it came out kind of strangled, so what? Steve turned back around slowly, and Tony looked down from his name on Steve's chest just long enough to note that Steve was hard too. "You want this."

"Want what, Tony?"

"You have my name on your chest." Steve swallowed, and he seemed the realise what Tony was saying because he nodded slightly. "You want me to take you, to own you. You're showing everyone you're mine."

"Yes."

And that was the last of Tony's control gone. He was up and stalking towards Steve before the other man could blink.

"JARVIS, lockdown."

"Of course, sir."

"On your knees, Steve," he said, and Steve dropped. Tony immediately clutched his hands in Steve's hair. "Mine."

Steve reached up and unzipped Tony's jeans. He was going commando, of course, and he was fully erect. Steve leaned in and lapped at the head, licking at the pre-come that had gathered there. Tony's fingers tightened in his hair. His hips bucked, and Steve took as much as he could, letting Tony set the pace. Neither of them could tell you how much time had passed when Tony pulled out and away.

"Going to fuck you. Make you mine."

"Yeah." Steve stood up and turned, bracing himself against a conveniently located wall. Dummy rolled over, a tube of lubricant in his hand, which Tony took before shooing the bot to the charging station. He pulled Steve's jeans down revealing the tighty whiteys that always made him smile. They just fit so well with who Cap was. He lubed up his fingers and then leaned in, biting down on Steve's neck as he rubbed between the globes of Steve's ass. Steve pushed backwards and Tony pressed his middle finger inwards opening Steve up as fast as he could. He needed to be inside him. He needed to own him. He hadn't realised how badly he needed Steve to be his until today.

"Never get to leave me. You're mine. Mine. Never letting you go."

"Yours."

Tony pounded into him, and he felt Steve come, his cock untouched. He had a very sensitive prostate, as they had quickly realised. Tony loved that, he loved playing with Steve's ass, seeing how much he could take, how quickly he could get him to come. Five or six thrusts later, and Tony was coming as well. They slid down the wall, and curled together on the floor in a sweaty heap, Tony's softening cock still inside Steve.

"Well," Tony said after a long moment, "that was something."

"Sure was." Steve pulled off Tony with a wince, and rearranged them so he could hold Tony in his lap. "What was that about?"

"I don't know."

"Tony."

"It was the t shirt. I didn't even realise how much I want you, how much I need you. But then you walked in with my name across your chest as though you were telling the world you were mine, and..." he shrugged.

"I see. I love you too, Tony."

"Oh." Because, yeah, Tony loved Steve, that was a given. But that was what this was? This urge to claim, to protect, to comfort... okay, yeah. He could see that. They lay there on the floor in each other's arms until Dummy doused them with the fire extinguisher.

And if the next day, Steve dragged Tony back to their room and held him down while he fucked him, then that was only fair. Especially since Tony was wearing a t shirt that had Steve's shield made of Tetris blocks on it.