Of course its a mental illness.
My brain has never asked me to click onto a pornographic image of a child. Therefore my brain is normal, in that respect anyway.
If your brain asks you to look at pornographic images of children and you are then getting turned on from those pictures. Your brain has a problem, in that respect.
They are always talking about murderers being 'completely normal' because they are in every other respect apart from the fact they like to stick a knife/ shoot a bullet/ strangle/ rape and kill etc. people because they get off on it.
You would be suprised how many people ask for help before they know its too late
If someone has not committed any offence to a minor prior to images etc. being found then they have no right to go to prison, they have the right to be helped.
Thats my view anyway.