When Tony Stark had seen Natalie Rushman, his statement of "I want one." Was only half joking. But there had been Pepper and other circumstances. And then he found out she was working for SHIELD and he wrote her off the list. Natalie Rushman was a yes, Natasha Romanov was a terrifying no. He hoped, and expected, to never see her again after everything was over. He was rather foolish in that.
They were both told they were borrowing trouble when it came out. Despite that, the team supported them. And Tony was pleased when Fury nearly had kittens over it. Despite all of that, they made it work. They fought, sure. Any healthy couple fought. But they resolved things fairly quickly, usually with make-up sex. Which neither party objected to, of course. Still, it took a while before much of it really meant anything.
When Tony moved the team into the Tower, he expected chaos. He expected destruction. And he expected a distraction from the problems he was having with Pepper. And he got what he had hoped for. And so much more. A year into the whole Avengers business, and Pepper had broken it off, they were still friends, and Tony was not about to displace the best CEO Stark Industries ever had. Besides, he had his little boy band to work with. He still flirted shamelessly with everyone, and never, ever expected to be taken seriously. And Natasha's only prerogative was to shut him the hell up. What started out as a simple forceful kiss to shut Tony Stark's mouth blossomed into something neither of them expected.
The gradual growth in their relationship creeped up on them, really. A year into the whole thing, two years as team mates, and Tony first blurted out something that made Natasha pause. The word love had never been bantered about between them; pet names, endearments, yes, but never the word love. And Tony's expression turned horrified after the words had come out, but Natasha's expression softened and she returned those very same words. It marked a significant change in their status.
They tried to keep it hidden, at first. Because it was so new, so fragile to them. And neither had the best track record with relationships. Hidden kisses here and there, Natasha sneaking into Tony's bedroom at night. No one ever caught them, they ended up mutually deciding to tell the team. But only after a terrifying incident that put things in perspective for them.
They never discussed children. They just were not suitable to be parents. They had each other, they had the team, and that was enough for them. And for all the teasing and banter, they were happy with their lot in life. Besides, with the life they lead, it just wasn't possible. They often joked they had a family in the Avengers, children and siblings both.
It started out with Natasha going on a solo mission for SHIELD. Tony didn't like them, but he respected her choices. She was twice his age, he knew she knew what she was doing. But that did not stop the initial panic when Fury told them a week later that they had lost her somewhere in Russia. They were still keeping their relationship a secret at that point, and he barely held it together and the team mobilized. It took every bit of his strength to hold himself together and not let anything slip. It took them two fretful days to find a location.
They developed rituals, patterns and such together. Tony learned how to read Natasha even when she did not want to be read. And Natasha learned very, very quickly how to read beneath Tony's masks. Neither claimed to be whole people, they knew themselves and each other were broken. But they made the best of things. And it worked.
Tony barely kept it together as they stormed the compound. He all but ignored the rest of the team as he frantically searched for Natasha. He found her first, and sincerely wished he hadn't. He found her huddled in a cell, beaten, bloodied and bruised. And drugged, because she would not wake when he touched her. Which she always did. He tried to keep his voice level when he told the team he had her. He tried to keep a level head when he assessed the best way to carry her.
They found unusual things about each other. Tony found that Natasha, much to his surprise, was a very tactile person with people she trusted. She found that he was the same. And it worked out well for them, for all that Clint teased them for cuddling. Neither really cared, it made them happy, and that's what worked for them.
When he walked out of the building cradling Natasha in his arms, he was given a few funny looks, but he ignored them. He nearly didn't relinquish her to the medical staff. He didn't want to, but he knew he had to. And he stayed by her side when he could. And when she woke, he blurted out the words that they had kept from each other. And she smiled and returned them. And that marked the biggest turning point of their relationship, and the very same day they shared it with the rest of the team.
It wasn't until a few years later that they decided to make it one hundred percent official. A small ceremony that included only the Avengers and their significant others. And they kept it well out of sight of the public. The general public had no clue about their relationship, which suited them perfectly.