But how much harm could they do in trying? That is the question you should ask. Look up Anthony Comstock if you want an example.
How certain are you of this? Would you be able to successfully defend every photograph or email you have ever produced against obscenity charges? I think, if it came down to it, the courts could find a hell of a lot of inadvertent pornographers out there.
Note I said 'could': Selective enforcement of over-broad laws is a boon and a horrible threat, because it implies everyone is guilty of something, so the government could pound anyone who bothers them if it chose to.