Most of the time Wufei honestly barely remembers that Heero is a woman.
Women are weak. (He doesn't even really believe that absolute, anymore -- Meiran and Sally, and even Relena, in her way -- they showed they weren't.) Heero is the strongest person he knows, stronger than him, better. A worthy adversary, a foil.
Women are nicer to look at. They take care of their appearance, at least. Heero wears old jeans and too-large jackets, and her hair is a bird's nest, cut when it starts tickling her neck and otherwise left to its own devices.
Women are visibly different. Heero's flat as a board and has no hips to speak of. When they were younger people routinely thought her a pretty boy, and she never bothered to correct them. (Duo used to tell entertaining stories about her and Relena's first meetings.)
Women are... Wufei doesn't know, he isn't sure how to pinpoint it; they're different, the way they think and act and speak, of course they can have valor and strength and smarts and sarcasm and violence as men do, but even so, even then. He can usually tell male from female merely from ten lines of correspondance; there are tells. Heero doesn't have any.
Heero is... just Heero.
"Tell her you're dating me," she says, smirking faintly, and even as he blusters and growls a part of him says, 'oh.'
It's stupid to get involved with a coworker, and stupider with your partner; not that he thinks Heero would lose objectiveness, or that *he* would -- if he dropped the ball on a mission to protect her, Heero might kneecap him herself ... no, she wouldn't, but she'd do worse, she'd stop respecting him and pull away -- but there are regs. They fit too well to let something like this force Une to reassign them.
But a part of him says yes. It would be weird. It would not be husband-and-wife, clear defined roles and patterns to conform to. Heero would not let him protect her and she would not know what to do with the classical trappings of romance and he can't even imagine wrapping an arm around her shoulders to keep her warm; she could stand naked at the South Pole and it wouldn't occur to him. He'd probably just make a snarky comment about her frozen toes and she would snort at him and steal one of his pairs of socks. It would be nothing like what he had with Nataku.
He thinks maybe it's as perfect as it gets.