I agree with everything you say, and have developed a special radar for those who are hiding behind political correctness and "common sense" (if it is "common", there is very little "sense" in it).
If you say that a two year old can learn to spank the monkey on her own, then I have to retract my statement that that is impossible. Since I am not a girl and have discovered my sexuality a few years after the age of two, and you say you did it on your own that early, then I can only conclude that it is indeed possible, provided your observations are perfectly correct (e.g. that you were not shown how to do it for instance or were intentionaly or accidentaly guided by someone to it - through images, touching or who knows how).
On the other hand, in my vocabulary, even just an interest in child's sexuality by someone who should not be so intimate with the child is a form of abuse. Maybe a more appropriate wording is "manupulation" (which is a form of abuse). An attempt at that happened to me when I was a child, and although no physical abuse took place, there was a clear interest in my sexuality by someone a little older then me (a child also), and was interrupted by accident. When I told my mother the whole thing, she went ballistic (not at me though) and explained that my privates are only mine and no one elses business. Had that "play" continued, I don't know what the result might've been. Maybe I'd start playing with yourself, spanking the monkey early too? Would that be OK? Don't know... but I am all for natural rather then forced or manipulated.
Even sexual interest expressed by other children of the same age can be dangerous: we as parents cannot know what kind of activity or behaviour the other child has been exposed to and as a result considers normal. Our child then may learn that type of behavior and as a result do it as well, when the said behaviour is not a good thing (whatever the behavior is). Even though my kids never went to preschool, I did notice that they were still bringing stupid psycho-games from the playground which they quickly learned even while my wife, their mother was watching and trying to keep them away from those troubled kids.
In the end, abuse has many forms, and it is not a bad idea to be super cautious in that respect. I am paranoid when it comes to child abuse because I have seen how easily it can happen and it has happened to MANY people I know (and was mostly imperceptable).