[pyre]

I doubt that viewing sex is harmful to children, but most porn is not in the realm of reality. As long as children are able to realize that porn is not reality, there should be no issue, but if children are introduced to sex through pornography, then pornography becomes what they believe reality is.

(I should note that, though I am not in the UK, I do not support this action. "Nanny State" indeed.)

{edit=expanding} I think it's all about how children relate to others. Violence definitely has an effect on children, but I think it may have less of an effect on them than viewing pornography does. Why? (well, let me answer that question that I brilliantly asked myself!) Because most of the elements of the violence that you see (I'm not talking about sexual violence here) in media are elements of relationships with other people that most children have already experienced by the time that one would think that they are able to view violent media. Basically they have experienced the reality of relating to people in such a manner, and can easily distinguish the fantasy (e.g. getting angry at someone, vs getting angry at them and then beating them with a baseball bat).

On the other hand, pornography is all about relating to others sexually. Children have no experience with this, so the viewing experience becomes not just entertainment, but education, and I don't think that most porn has life lessons in it that we want to teach children.

Note that this is a very general argument. Even the word 'children' is almost too general to mean anything. A high-schooler and a 3 year old are on very different levels with respect to how they can handle these things.