I think it has a lot to do with, as you mentioned, the potential for psychological and emotional harm from incorrect decision making with regard to sexual activity.
Yes but as I said, this may be largely because we are trained to believe it is wrong. A child who has been sexually abused may believe their is something wrong with them and there upbringing because they have been taught to believe this by society. Would this necessarily be true if society had a different view? It is similar to homosexuality. When it was almost universally believed to be wrong, people that were homosexuals suffered psychological harm because of the social stigma attached to it, but that does not mean that homosexuality itself is psychologically harmful.
Also as I conceded most sex with children does take place within abusive relationships and it is indeed disgusting and wrong. It is no wonder children who are brutally raped by their parents suffer psychological harm. What I am saying is there are circumstances in which psychological harm may not occur. Remember by sex I don't necessarily mean penetrative sex, but all forms of sexual activity.
Nature itself delimits a time frame from which sexual activity is feasible through our sexual development. This of course varies from child to child. Some are not capable until 14 years of age, others are as young as 10.
Again this is only true of penetrative sex. It is common for young children under 10 to masturbate and touch each other genitals and to get pleasure from it.
As human beings, the general consensus of the innocence of the child is derived through developmental immaturity; emotionally, mentally, physically and sexually. Ultimately, these natural phenomena have defined the very notion 'childhood'.
Why is this nebulous concept of "innocence" important?
The question is more contentious in considering children of the age of 10 upwards, during the ages when their sexual desires start to come to fruition. However, I would say that the harm principle is the guiding factor during this period; and rightfully so. An adult can decide about the education, diet and sporting habits of their child. However these are generally decisions made to assist in the development of the child, and are made with the best interests of the child in mind.
Yes, and who is to say that allowing the child to participate in consensual sex with an adult could not also be in the best interests of the child?
Foisting sexual activity on a child incapable of making the decision of their own accord is particularly self serving on the part of the adult. It does nothing to benefit the child, and is generally undertaken to satisfy the urges of the adult.
What if the child does make the decision of their own accord?
You are generally correct. Don't try to misinterpret me as saying sex with children is always or even often okay. I'm just pointing out that it is not necessarily, always wrong, which most people seem to assume it is.