I talked about this when Newsarama was speculating that she might be his mother. In the original scenes of Tony and Natasha’s first meeting, sex was never implied— their one dinner date was just a distraction for her partner to sneak into Stark Industries. Deadly Origin, which was released to cash in on Natasha’s appearance in Iron Man 2, absolutely seems to retcon things so that Tony and Natasha had a much longer affair, which would imply they probably had slept together, but it isn’t explictly shown. I prefer the original verson, myself.
Tony and Natasha definitely did sleep together in a bad 90s Iron Man comic, though.
Natasha: For that, capitalist dog, you shall feel the Widow’s bite.
Tony: Mmn. Don’t mind if I do.
Panel from Iron Man #317.