There is no justification for ciminlising the human body. If one was to be naked in their garden, on a beach or secluded woodland one would be a criminal. These are victimless crimes.
In the same way being naked in a populated area should not be criminalized, what right dose anyone have to be 'offended' by my body? If I told someone I was offended by their arm and that they should cover it up it would be obsurd. We critisize muslim societies for forcing women to conceal their faces.
Children are not harmed by viewing nudity – many tribal societies are complelty naked, yet they have a more realistic, healthy view of the human anatomy than 'civilised' societies. Prudes should not be allowed to criminalise ones natural state of being.
As for flashers and those who would use nudity to offend and harm people, these people can be prosecuted with being 'naked with the intent to cause offense/harm'. A man waiting naked outside a school, thrusting his genitals in the direction of the children is not the same as a nudist like myslef walking down the street to do some shopping.