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It didn’t happen the first time they had sex, or the fourth or even the twelfth.
(Yes, Tony was counting. He never thought he’d get to have this, so fuck everyone else, the novelty hadn’t worn off and likely never would. Really, had they never seen Steve naked?)
Granted, the twelfth time was only about the fourth day after they’d started fucking – Tony loved sex and Steve shared his interest wholeheartedly – but they hadn’t noticed anything odd. To be honest Tony was too far gone in love and lust to notice any such minor changes, and Steve was too far gone in everything Tony. It wasn’t until the second week that Steve noticed he could scent Tony when he wasn’t in the room.
They put it down to his super-senses locking onto Tony’s slick now that they were intimate and let it go.
When Steve started marking his neck, they thought it was a newly discovered kink for Steve that Tony was vocally enthusiastic about.
When he started being more handsy and aggressive, Tony thought Steve was finally becoming more sexually confident in their relationship and cheered him on.
For the most part, all of that was true, but it wasn’t until their second month together that the final piece of the puzzle came together to form a different picture.
Tony’s legs were wrapped around Steve’s waist, held up by the wall and Steve’s hand under his thigh. Steve was currently trying to smother Tony’s moaning by shoving his tongue down his throat while fucking into him like it was his life’s purpose. Tony’s body was on fire and in a distant part of his mind he knew his thighs were cramping but that was completely overshadowed by the glorious sensation of Steve’s cock pounding his ass.
He felt stretched and used - it was their third time today, Tony would never run out of praise for Steve’s libido - and his mouth was running off without him, mumbling hoarsely, “Fuck, Steve, please, more, please.”
Steve thrust in deep and stopped, then nibbled at his jaw before moving down to his neck. Tony definitely did not whine at the interruption and tilt his head for better access.
“I can do this all day, Tony.”
That image – the sensation of Steve filling him so perfectly, surrounding him with his arms, his muscles, his scent, his Steve-ness – brought him to the brink. “Oh, fuck, yesplease. All day – don't fucking stop, dammit!”
Steve growled and bit down, fucking him harder and Tony was gone, shouting his release and clenching down and coming all over Steve’s stomach and chest.
“Tony, tonytonytony-fuck!”
Tony came down just in time to see that furrow between Steve’s brow, and felt a sharp pain at his hip that would probably be a hand shaped bruise tomorrow as Steve came inside him.
He was kissing his way from Steve’s mouth to his ear when he felt it. Something he never thought he’d feel again after leaving his universe and falling for this Steve.
A knot.
An Alpha knot swelling in his ass.
Even as his body responded to this unexpected yet intimately familiar pressure, he couldn’t help but think, “Oh. Well, that’s a different puzzle altogether.”
~~
Fin