I think you have cause and effect confused. The man isn't a rapist until he rapes her. And if he -- like most men -- would inquire into why a woman was roaming the street naked, I think one could reasonably expect that a civilized human being would respond to a woman in need not by raping her but by offering her safety. On the other hand, a woman who was strutting around in public in her union suit has no moral high ground from which to accuse men of presumption, or to condemn them for wanton acts.
A woman parading around in public should not necessarily be a candidate for rape, nor should such a rape be considered innocuous. But her nudity certainly creates an extenuating circumstance, and could even preclude a conviction. On the other hand, it is clearly a violation of decency for her to appear naked.
And to equate common modesty with sharia is not only an ad absurdum argument, it is a childish oversimplification.