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She knows most of S.H.I.E.L.D thinks they're fucking.
Natasha doesn't blame them, it's what most people seem to think a partnership entails if one or both halves of it are female, and she's no stranger to assumptions. In her cynical moments she wonders bitterly if people say the same things behind the backs of male partnerships.
But Natasha is the best at what she does and when she says she can have anyone she wants, it's not out of conceit. And it isn't like the thought hasn't entered her mind. She's noticed his body, of course, as perfect a weapon as his bow. She knows he's noticed her curves, because what kind of man would he be if he didn't?
They wouldn't make love on satin sheets. It would be against a wall, barely after a mission or even still during it, and he would shove her shoulders against the bricks while she bit his mouth, and the strap of his quiver would dig into her chest like a constant reminder of who they weren't.
They never discuss their personal lives (as if she had a life other than him), but when Clint makes tea hers is always black, strong and with a hint of sugar, just the way she likes it. His favourite biscuits suddenly appear in the staff kitchen and when, biscuit crumbs on his lips, he catches her eye across the control room, he smirks and at that moment, she imagines what it would be like to kiss him.
He would taste of death.
So does she.