The thing is, Natasha really only has two kinds of relationships.
First, she has the kind of relationship where she's after something, and she's determined that the most efficient way to get what she wants is through judicious application of her rack. Which...probably sounds a lot worse than it is, honestly, but what the fuck do you even want from her, she's a secret agent, it's part of the training. It's tactics, it's lines of sight, it's what she lets them see and what she encourages them not to notice. It's lowered eyes (so they can't see what she's watching) and innuendo (so they forget to ask why she's there so late) and it's skirts cut high enough to be a distraction (and for good range of movement).
She's not always sleeping with people for intel - just, sometimes it's to her benefit for people to see her as a very attractive woman, first and foremost. They underestimate her, because they're sexist dipshits, and then she can take them out, and sometimes along the way she gets an orgasm or two out of it, which is pretty okay in her book.
(This, for the record, is what goes down with her and Tony Stark, when she's Natalie and not Natasha - not that she sleeps with him, but she's certainly prepared to. She sets it up so that he focuses on her charms instead of the muscles in her shoulders and her thighs, and he is more than obliging. It gets the job done, they save the world, she makes her report, he and Pepper Potts get their act together, and that's that.)
And then she has the other kind of relationships: the kind that are purely about the release of stress and tension, like stretching or sparring or getting a massage - something that's just for the fun of it, for the feeling of her body doing something she's damn good at. It's not emotionless - she pretty much has to like somebody to fuck them for fun - but it's also not fraught with tension and longing. Bodies do what bodies do, if you give them the opportunity, and it's fun for everybody concerned, so there you have it.
(And this, for the record, is what happens with Pepper Potts, late one night when Stark's locked himself in the mansion, throwing together bits and pieces of untested science and driving Coulson up the wall. Pepper flings down the stack of third-quarter projections and lifts her arms behind her head, tugging on her elbow until the joint of her shoulder makes a soft pop, and Natasha looks at the curve of Pepper's breasts against her dress and thinks, yes, absolutely.
Afterwards, Pepper zips Natasha's dress back up for her and kisses her cheek, smiling softly, and goes back to the projections with her eyebrows quirked. And then Stark invents an entirely new element, because he's an overachiever, and almost dies, because he's an overachiever, and Natasha incapacitates a bunch of guys with her thighs, because she's an overachiever, too.
Stark and Potts kiss on the rooftop; Natasha smiles up at them and goes to finish her report.)
So she has two kinds of relationships - or at least she used to.
Now, though, things seem to be changing. Stark still stares at her tits, even though he knows who and what she is - but then he finds her in the gym and lets her take him out with both hands behind her back, and just laughs when she pins him to the floor four times in a row. Now, Natasha has a standing coffee date with Pepper Potts, where they talk about everything and anything, where Pepper smirks over her coffee and rubs her ankle against Natasha's under the table.
Now, she has Stark's hand on her hip, pulling her against him, and Pepper's lips on the side of her neck, just a hint of teeth, and the sneaking suspicion that she's been outflanked - and masterfully so.
She thinks she's going to enjoy it.