I know that I'm certainly aware...but I guess I don't count seeing that I'm a member here (though very new)
But I've had a few conversations with buds, "average joe's" if you want, and they've all made it clear they're aware...and we all seemed to agree that it was a combination of not caring and not minding (or wanting) the attention.
Though once I remember introducing my friend Dylan to a new pledge and Dylan asked him "Are you Jewish?" and he said "No, why?" and Dylan pointed at his crotch, where you could see the outline of his dick quite clearly, and said "You're circumsized." The pledge blushed a bright shade of red, noticeably very embarassed, and self-conciously attempted to adjust his pants from clinging to him for the rest of the time we hung out.
So I guess it's possible some guys have no clue...maybe sometimes it's just a clothing issue they don't notice at first... like forgetting to zip up their fly.