They shouldn't be banned. However, that's only because it's more productive to campaign on it, and get the Sun to recognise the sexism of Page 3, how inappropriate and anachronistic it is and remove it voluntarily than for the state to enforce such a decision.
(That being said, I would not be opposed to the paper being banned on public property especially where children may well be present such as parks, schools or transport at the discretion of the appropriate local authority and with private businesses encouraged to follow suit).