This is a basic matter of civil liberties and the right to a private life. The fact that someone is a policeman or a teacher (or other public sector worker) shouldn't give the state the right to discriminate against them in their private lives and impose special rules banning them from what everyone else can do perfectly legally.
The police can't be members of the BNP. Teachers can't moonlight as strippers – or they get the sack! Why shouldn't these people be allowed the same freedoms as everyone else when they are not at work?