I think it's fairly straight forward.

"Natural" in such a statement is perceived as what "nature" intended as "normal" for human relating. Basically, a man copulating with a woman to produce offspring... furthering the survival and endurance of the species.

Add what ever religious overtones you want or not... but in "nature" things mate, have babies, survive.

"unnatural" then becomes the opposite of this. So sexing a child has no procreative aspect. Further, it may even so emotionally/mentally damage said child as to make them ineffectual at life and having "natural" relationships.

The assumption is that sexing the child will harm it and thus go AGAINST the nature of nurturing children and furthering the survival of the species.

I don't think it's just a matter of being "uncomfortable" to them. I think they perceive that actual harm will occur.

Logically, it follows that if they think child sexing is harmful then anyone who desires such a thing is also "unnatural" whether they engage in the acting out of what they want or not. In the media, the popular opinion is that pedophiles WILL always act out on their sexual impulses.

The statement does show a gross ignorance on the whole subject and dynamics of pedophilia... but (/shrugs) what's new.

Blessings
Cat.