Pornography is harmful in the same way that cigarettes and tobacco are harmful, and overeating, and driving too fast, and so on and so on.
It is absolutely not a 'yes' or 'no' answer because it is a complex phenomenon, driven, as are so many worldwide pleasures, by money, in amounts that make the Hollywood film industry look like one film student with a steadicam.
There is serious money to be made for the stars who manage to hit the career path that leads to that end - but of course at the other end are seriously exploited and abused people - there is no simple measure for everything.
I would never subscribe to the notion that pornography alone leads to the criminal activity and abuse that includes rape and child abuse.
Rape is not about sex it is about robbery with violence, and child abuse is about control and subversion - and both are driven by far deeper negative personality traits than those normally satisfied by basic pornography.
It is simply too wide and complex a situation to say that it is this, or that - life is rarely that simple.