Does porn cause harm? That it does is indisputable. There are many cases where men report decreased sexual performance as a result of watching too much porn (that is a harm particularly if it interferes with close intimate relationships). If nothing else, that is a harm. Does it cause harm on the balance? That, however, is unknowable.
But the harms which are readily documented don't match the harms which are posited. The harms which are posited are also readily observed but a causal connection is impossible to draw. Our culture has for the better part of a century (since probably the 1920's or at least 1930's) slowly been decoupling sexuality, marriage, and reproduction. Pornography may be seen as a consequence of that separation, but it certainly cannot be seen as the primary cause given the timing. Whether it furthers and further entrenches a pattern of hedonistic sex may indeed not matter -- it is only one small piece of a much larger cultural pattern.
So in the end, I don't know if it causes the harms portrayed. What is clear is that a failure to deal with the context rather than the technology is a much larger problem.