It is an interesting issue from a legal perspective.
Our criminal laws are for the main part divided into 'moral code' laws - don't steal, don't hit people etc- where the law will only go so far as to impose a prohibition. Ie, X is morally and socially repugnant, so if you do X we will punish you. For the most part there is a social consent that such behaviours should be prohibited.
The theory is that the law will only intervene to say what you should not do, not what you should do, ie, it will lock you up if you are abusive to someone, not if you fail to be nice to someone. It had no need to tell you what behaviours are socially expected, because society will do that itself. Ie, if you are not nice to people, society will impose it's own punishment in that you will lose friends.
The other category of laws are based on health and safety or are necessary for the functioning of society - wear your seatbelt, wear a helmet when you are on a motorbike, pay your taxes. It is only in such cases that the law feels it necessary to step in and proscribe behaviour, on the threat of punishment if you don't comply.
The problem with the offence of outraging public decency (when it is used to punish behaviours that do not fit elsewhere into a criminal offence) is that a) as this thread shows, there is seldom consensus as to what people are outraged by; and b) in this case it is used to proscribe behaviour.
We are all naked in our natural state. There is no real agreement as to how much clothing we have to wear in order to be 'decent'. Some people are offended at seeing a naked body out of a context in which they are accustomed to do so, some people aren't. Even those who are offended are not actually harmed, in the sense that it does not affect their own physical integrity or prevent them from living their lives as they would wish. They are not left with any lasting fear or distress (other than temporary social discomfort).
The law does not generally intervene when we are offended - ie my friend told me I lazy at housework - because it is not seen as being serious enough for the law to intervene. It is something that can be dealt with by my never inviting her to my house again.
The law of outraging public decency is an old one, and it is an anomaly in that it is used to proscribe a moral code in a way that the criminal law does not normally intervene. That is why it is so problematic.