I dunno about squeaky-clean and innocent, really -- I mean, Steve DID grow up in a big city, tour with showgirls for months on end, and serve in the army for at least a year, and he can be awfully snarky underneath his courtesy. But yeah, he doesn't seem to think of himself as a person other people might desire sexually and/or romantically, so I doubt he'll guess Natasha's intentions without the full twenty questions and a direct physical advance on her part. :-)