Got to agree with Dexi and a few other ladies, your reaction seems a little antagonistic. Dunno just seems a bit strange to get that indignant about someone showing an interest. As for other examples, a random stranger would not asked someone for medical or accounting advice unless they are aware of their professions. Since most of us probably don't walk around with a massive scarlet letter embroidered to our clothes, how would a guy know when meeting us in a social setting?
Dunno the context of what these guys say to you but while I get compliments and get chatted up, I seem to not get outright crass proposals.