" So you believe, that as long as no physical action is taken against a child there is no harm done? "
No, I clearly don't, which I why I wrote "Clearly, the world would be a better place if child pornography and the hideous crimes that are carried out in its creation were stopped.". I'm not sure how much clearer I can be than "hideous crimes", but perhaps the chaff from the strawman you are constructing is making your eyes water.
"Would you want your child to be with someone who although apparently perfectly harmless, is visualising your child being raped by him, just like in the pictures he downloaded last night?"
For me this as a very big problem and it is damaging though it may not show a bruise."
Damaging to whom? I'd argue that the distortion of relationships between adults and children by the fear of abuse is also harmful, and for every child being somehow harmed by the unacted thoughts in an adult's head, there are tens or hundreds being affected by (for example) the almost complete removal of men from primary education, and the feeling that most adults, male or female, do not offer help to distressed, lost or otherwise threatened children that they are not related to. The damage caused by unacted fantasies is extremely hard to quantify, if indeed it exists at all; the damage caused by the distorted fear of abuse is real and tangible.
Simply assuming that everyone who doesn't share your precise concerns is somehow defending abusers is unhelpful. Child pornography, its production and its market, is a cancer in our society. However, it is not the only problem we face as parents, and the desire to protect against every possible risk is not itself a risk-free operation. If one consequence of a fear of the (small) risk of child abuse is that long-standing activities that rely on the trust volunteers stop happening, that is not something to lightly dismiss.
Protecting children against actual physical harm is very important, although it doesn't trump everything else (as otherwise no-one would ever ride a horse or jump on a trampoline). Protecting children against the possibility that someone may look at them with evil in their heart, although that evil is unacted, comes a long way down the list of priorities.