I lived in an enormous city in Japan so take this observation with an appropriate quantity of salt, but I have not seen much in the way of porn in my environment since returning to the U.S. Keep in mind that I have walked around San Francisco - even some of the seedier areas - but I haven't perused any red light districts. It does not appear to be common here for men to openly read their girly magazines in public, at least not in my experience.
In Tokyo, I didn't have to go anywhere special to see men looking at naked women. They did it on the train while commuting. Such magazines were sold on the street, in book stores, and in newstands. While I really don't care what men look at to titillate themselves, I think that such behavior (like using the bathroom or having sex) should be private and I should not have been a non-volitional witness to their prurient interests.
I still don't miss seeing men looking at porn and such materials being sold everywhere as I did in Japan.