I hear people say that all the time, as though it were established fact. On what do you base this statement?
I've noted that liberals and conservatives share one particular fault: no respect whatsoever for the intelligence or responsibility of their fellow citizens (of course they always exempt themselves from this judgement). They speak all the time about morality, yet seek to arrange society so that morality doesn't matter and no-one has to take responsibility for their choices, by removing the objects of "evil"--be they guns, drugs, dirty magazines, whatever. A "good" choice which is forced upon someone is not good at all. And objects like guns or dirty magazines do not possess some magical power which turns good people into evil people.
Ever heard of concrete moral truths? They exist. One of them is is that pornography helps create a moral climate conducive to sexual abuse and exploitation. It reduces sex to recreation and human beings to mere playthings and machines. It denegrates the sacredness of marriage, family and human life.
Even those who claim porn is harmless must tacitly acknowledge the power and influence of words, pictures, and ideas. Clearly what we see and hear affects our thinking and influences our behavior--and porn does indeed have a damaging impact on conduct toward women. Ideas do, after all, have consequences.