Can't porn censorship justify political censorship later on?

A post by VP Abtahi about the desire of some open-minded Iranians to have a porn-free net access for their children, and a brief discussion about it on my Linkdooni, has created a new ethical/policy-making dillema for me. The problem is very delecate:

How can you prevent the desire of some people to keep their children off Internet porn from becoming a good-looking justification for the political censorship by the governemtn? In other words, how can you defend the freedom of speech on the Net where people are quite supportive of the censorship on erotic content?

Note that unlike the West, child pornography is not an issue yet in Iran. Instead, children's access to porn is more important.