>Prepubescent children are physically incapable of fully understanding what sex is, almost by definition.

I doubt this. It is certainly not controversial to claim that children 8 and up can have romantic feelings (puppy love). Sexual intimacy doesn't by default mean penetration (for which could reasonably be argued that it is physically unpleasant).

An incapacity to understand fully what something is doesn't automatically make it bad. A prepubescent child might not fully understand why it should go to school, put on clothes when going outside on a summer's day, not shit in the backyard and so on, and you as a figure of authority disregard the child's ignorance about the full rationale for doing and not doing these actions (among others) and force the child to do them (sometimes under the treat of punishment). Such actions are not generally taken to be wrong, bad or abusive.

Furthermore, the examples above are limiting in nature. They restrict the child's desires, urges and inclinations. In the case of intimacy with a child, assuming the child isn't obviously coerced into doing so, it may well be in accordance with the child's wishes (to feel loved, to have physical contact and to have romantic interactions) and thus not limiting in nature.