Natasha Romanov doesn't trust and she doesn't love. The only people who have come close to breaking that rule are Clint Barton and Phil Coulson.
Natasha sort of had to trust Clint when he gave up the op for her, risked his career to bring her in instead of kill her. She had a little intel on who and what she was joining (she's the Black Widow) but not a lot (they're SHIELD). She had to trust Clint to fly the plane, to do what he said he would do, to not just dump her in a cell (not that she hadn't gotten out of cells before, but she was just so tired of never being able to relax, and if this turned out to be a kidnapping and not the rescue Clint promised, she wasn't sure if she cared enough to escape again). And Clint, surprisingly, pulled through.
Natasha's respect is easier to earn. Clint earned part of it by being a man of his word, more of it by turning out to have gone behind the backs of just about everyone he worked for to bring her in, and the rest by being just about her equal in a fight. There are about a thousand elements to a fight, of course, especially if one considers all possible venues and conditions, and there was no way they'd be evenly matched in all of them, but where Natasha fell a little behind (she is only human) Clint picked up the slack, and vice versa (he's only human too).
The first night she fell asleep beside him, she had a small panic attack upon waking up-no one could tell, no one could see how her breath pulled in and just stopped until she forced it out again, no one could hear the terror ringing in her ears or her mind screaming what does this mean and if she kept her back to the walls, well, she nearly always did that anyway even though her situational awareness is superb and she doesn't need to see someone to know they're trying to sneak up on her, no one should have been able to tell, but Clint knew. Clint watched her without pity or fear, just sat nonthreateningly and didn't try to touch her while she parsed. Did she love him? Was that what this was? She read romance novels now that she had time to relax, they were stupid and required no energy to understand, and that was a thing they talked about, complete trust, and mutual understanding, and having each other's backs and appreciating the small things, and didn't she have all that with Clint, didn't she understand him and he her and hadn't they had each other's backs since they'd met and she hadn't gotten a night's sleep this good since she'd locked herself into a room with reinforced walls and no windows in a part of the world she wasn't supposed to be. When she'd calmed down and thought through everything at multiple angles, she allowed herself the tiny betrayal of checking the time to figure out how much she'd lost. Only Barton knew, and Barton wouldn't use this small weakness against her. He raised his eyebrows the tiniest amount, as if to ask what conclusion she'd drawn. Moving slowly and deliberately, Natasha got to her feet, crossed to Clint, and kissed him.
It was nice, in an odd sort of way. It didn't make her want to go further, but it was nice. She broke it off, met his eyes for a few seconds, and then turned and went to begin her day as though nothing had happened.
They never had sex. Clint never asked whether she was actually asexual or traumatized, and she was quietly grateful for it, because she didn't know the answer. She wasn't even sure this counted as romance, all she knew was that she and Clint ended up moving in together, and it made life easier on both of them, made it easier to sleep when they could and take care of each other when they were wounded, and calm each other down after a bad op. They learned to enjoy having meals together (even occasionally cooking for each other) and keeping things tidy for the other's benefit and having someone to come home to.
If she allowed herself to become sentimental, which she doesn't, Natasha would say Clint is perfect for her. He understands her. He holds her up in all the ways which she doesn't actually need to be held up in order to survive, but which make her a happier, better functioning human. He doesn't push her, except when she needs it (which is still rarely). He can read her, most of the time, and if he could read her all of the time, she thinks she'd run anyway out of pure fear. He's whip smart and deadly competent, which she appreciates, as anything else would be boring. He undervalues himself to a devastating degree, but even this is good, because it gives her something to work with, a way to take care of him, a way for it to be okay that she isn't perfectly mentally healthy either. (It was the fact that Loki didn't know any of this that convinced her that Clint could be saved, the fact that it was her past as a mercenary that he tried to use against her that kept her completely in control and forced her to fake her tears, because if Clint Barton truly wanted to damage her, that is what he would have used against her and he didn't.)
She knew she wasn't the ideal girlfriend. She doesn't put out, but actually she's okay with that—if Clint needed that he could get it elsewhere. Less easy to reconcile with his calm acceptance of her as a package was her inability to be in the slightest bit sentimental, to express her gratitude, to provide what most people recognize as emotional support. So when she was actually a little surprised that Clint found someone else, she hated her complacency, because why wouldn't he leave her if he could find someone he trusts and cares for the way he does her, and who can be all the things to him that she can't? Then she actually listened to what he was telling her, and thought that, as there's no such thing as a perfect boyfriend, and hers had to have a fatal flaw she hadn't yet seen, it makes a sort of beautiful irony that the flaw would turn out to be this: Clint was polyamorous.
Natasha had seen it coming, to some degree. She knew Clint and Coulson flirted. She knew that Coulson had Clint's trust to the same degree she did—she didn't fault him for it, Coulson was the only other person she trusted, too. She knew they understood each other, and it was Coulson's job to have their backs. But she hadn't realized it might be love, for them, too, not until Clint had her sitting down in the middle of a talk about what it would mean to open their relationship enough to include Coulson.
"Have you talked to him about this?"
"No. If…if you were to agree…I thought that would be the next step. To talk, all three of us." He grinned a little. "Are you kidding? You'd kill me if I didn't talk to you first." She blanked her face, because even though he was joking, the idea that he only went to her first out of fear of his life was discomforting. "You're worth more to me than that," he added, smile sliding off his face, and since when could he read her when she was actually trying to keep him out of her head?
She agreed to discuss it with Coulson (how could she do anything else, if this was something Clint wanted enough to break that social rule, and if it had to be someone, she was glad, infinitely glad it was Coulson) and then they were sitting together in Coulson's office, and Clint was explaining the situation. Coulson, to his credit, listened all the way through, and then turned to Natasha.
"I'd like your take on this," he said quietly, and Natasha felt ridiculously grateful.
"We hardly have a typical relationship to begin with," she said honestly. "It would only be a little bit stranger to do this as well."
"What do you want from me?" he asked directly. Natasha looked him in the eyes for a second before she answered. "For you to take care of Barton, and give him what I can't."
She had never meant that in a sex-and-emotional-support way before, but Coulson was already the other half of Clint's support system. What difference would it really make? It was Coulson's job, more, his self-imposed mission, to take care of them both, and if that meant, well, this, maybe that was her ticket to not needing to give more than she could. Maybe this was how Coulson could take care of her as well, by acting as…insulation, she supposed (she must be spending too much time down at R&D) for their relationship. And it did the double duty of clarifying what Coulson was actually asking—she wasn't interested in dating him, too.
They could have been perfect if any of them had been really good at communication.
Natasha never really worked out if she was honestly resentful of Coulson and Clint's relationship. They fit like a glove, they were happy, she knew it. She could see the way they looked at each other, and she just wasn't built right to know if Clint gave her the same look (there are severe downsides to being raised to assume everyone wants to use you). And they had a fabulous sex life. All you had to notice was the small variations in the way they walked after a night together, not just from the muscle exertion but the pride, the ego boost that slipped into their walks. If that wasn't enough, try the way they were both in far better moods the next day. She was glad, she supposed, that Clint was getting laid by someone he trusted that wasn't her. She was definitely glad he was happy. She just wasn't sure if she, as a girlfriend, had anything to do with that anymore. They cuddled less, definitely, but was that because Clint wasn't spending all his free time with her anymore, but attempting to split it evenly between them? That was the question, she supposed. Was the change mostly Clint's balancing act, or was it a transfer of his affections he was trying to conceal?
She hated not knowing. She hated more the voice in the back of her head that asked her what she had to offer that Coulson didn't, what Clint got out of this relationship, whether it wouldn't be better for her to break up with him and let him be just Coulson's. She was the Black Widow, she wasn't supposed to be insecure.
Clint didn't leave her. Clint still ate with her, still took care of her after missions. Sometimes he got them both to come home with him at once, especially after hard missions, and they all took care of each other, and Natasha didn't know whether to be glad or embarrassed that Coulson didn't get laid those nights.
And then Coulson died.
Natasha buried any immediately reaction to this, and not for the first time was grateful for her training. She didn't tell Clint. She thought he could still fight, but she didn't want to risk taking him out of commission with grief, and he would never forgive himself if he was too crippled by it to take his revenge. They fought, and they won, and she took him home after shawarma and told him.
He sat there for a long minute and stared at the wall to the right of her shoulder. His face crumpled, and he began to choke. Clint Barton may have been an attractive man, but he did not cry prettily. He choked and sobbed and attempted to rip anything his hands found, and reduced his voice to gravel with the force of his crying although very few tears escaped his eyes. Natasha sat next to him until he wore himself out with exhaustion, then she got half a bottle of water into him and allowed him to fall asleep pillowed on her chest.
And finally, she allowed herself to process.
She would be there for Clint. That was a given. She would be there to force him to eat and sleep, to wake him from nightmares and to scare off Tony Stark. She was upset that Phil was dead. That was also a given. He had been a good handler to her, and he had taken good care of Clint in ways she couldn't, and she had to be upset over anything that ripped Clint to pieces like this.
Natasha knew she was no Steve Rogers paragon of virtue, so she pushed aside shame and processed the less logical, less admirable emotions. She was a little bit angry at Phil for getting Clint to trust him so much more than she could ever trust anyone, and then dying. She was a little bit frustrated with Clint for allowing that trust to build. And she was a little bit glad that she was here to pick up the pieces, (here to show Clint, she was not dead, she had not betrayed him, left him, she was far more difficult to kill than Phil Coulson, goodness knows enough people had tried) and a little bit glad that she no longer had to wonder whether Clint had any use for her in his life. She was all he had now.
Phil Coulson will not confirm that he allowed Clint to keep believing he was dead because Clint still had Natasha and, somewhat like Natasha, he believed his part of the relationship expendable. That's a good thing, because Natasha would have murdered him for it.