As a native New Yorker, subway flashers can sometimes seem as commonplace as subway maps.
The first time you see it, you're completely horrified and cannot conceive of such an event occurring in polite society in public in the middle of the day. Did that man just not only expose his penis on the C train, but also rub up against me? The second time, you're a combination of grossed out and pissed off, and by your second week in New York, you're kind of over even noticing it.
Sometimes, even as an old, hardened, dick-witnessing New Yorker, it still can catch you off-guard. I remember being particularly annoyed by an otherwise normal older-teenager sunning his cock at me down the steps of the Houston Street 1 train as I passed on the sidewalk, and berating him. Usually, the many New Yorkers who witness this are about as moved as they are by the sight of taxis, puddles or $15 movie tickets, but this particular woman was having one of those, "I'm mad as hell, and I'm not going to take the sight of your penis on the train anymore!" days.
Witness her public smackdown of this particular subway flasher- it's kind of cool how the other people join in and laugh at him: