"Simple images of naked children are generally harmless to the child."
IIRC even clothed pictures of children are officially in the illegal categories if someone regards the pose as "provocative". Most family albums - at least in previous generations - would contain something that could be judged technically to fall into that category.
The problem with human nature is that a person's ego is driven by rewards. Tell them that they are doing a good thing by finding suggestive pictures and they will get their mental high every time they read a picture in that way. Even if their emotion is purported to be disgust - they are still getting an emotional kick out of it. In extremes they deliberately seek out that which they can loudly claim disgusts them. One Chief Constable said that if anyone on of his specialist team started to judge pictures dispassionately - then he took them off the team.
Power over others corrupts - as most jobsworths behaviour will confirm.