Please show me where it says that the crotch has to be exposed. Because I'm pretty sure you're just making rules up.
And please stop vandalizing the tags.
Clearly doing something simple like looking it up yourself is beyond you, so here you go.
Public Nudity: The state of being nude or mostly nude (no covering of the breasts or genitals) in or at a public location or event where one can be seen by others. This is in contrast to nudity in areas that are not generally accessible to the general public without prior permission. http://danbooru.me/wiki_pages/6136
Public: Use for posts depicting sexual behaviour (masturbation, sex, etc.) taking place in a public location. http://danbooru.me/wiki_pages/8652