And it's always a man; I've never heard of a woman doing this...yes, I saw on "Cops" the women exposing their breasts at Mardi Gras (sp?) but I have never read a story about a woman exposing her primary sexual organs to someone in public.
I can think of at least a couple of reasons:
Men are pigs
Male genitalia is more naturally exposable than female genitalia (a woman can't just unzip like a man can)