There are some practices which are clearly exploitative (molesting minors for example), and government's role is to prevent them.
Most governments try to set themselves up a moral guardians for the community. It is human nature that many people will try to control others, and dictate their morality. Politicians themselves are the kind of people who like controlling others.
There is vague argument that pornography leads some people to commit crime. There is a vague argument that allowing some people to watch violent films causes them to commit acts of violence. Frequently people who are caught committing violent crimes are found to watch large numbers of violent films, but the link is not found to be causal it is merely statistical correlation. People who commit violent crimes are probably obsessed with violence. Why would we be surprised to find that they watched violent films?
Rapists may be found in general to have many pornographic magazines, but there is no demonstrated causal link. Both may be linked to a third factor (like an obsession with sex). Further the magazines may actually provide an outlet for frustrations, which reduce the propensity to inflict them on people.
We see waffling arguments about causing desensitization and so on from people who have ulterior motives for the ideas they advocate. Elites of one form or another, religious zealots, feminists and red-necks alike think it is their right to dictate what others see, hear and ultimately do and think. In doing so they are initiating force against those people. It is a form of slavery.
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