well, in your argument that "sex is natural" you forget that violence too is a natural human behavior. in fact it is a natural animal behavior in almost every species.
sure there are certain ultra-violent movies and shows that can do major damage to a child's mind, and seriously mess them up for life, but violence in general is not such a bad thing, its just nature.
but, i do agree with your views on sex. why do we shield our children from sex?
why, because sex is no longer just an act of procreation or even love. sex is a drug, it is a vice. there are so many cases of immature sexual encounters these days, that it really has gotten out of hand. two years ago, one of the local ELEMENTARY schools had an EPIDEMIC of gonorhea of the THROAT, because 5th grade girls were charging 5 dollars for blow jobs in the bathroom. This is not a joke. this is a serious report. and, this is not a bad neighborood, it is an average suburban town on long island.
now, ask yourself this, would you rather your daughter caught an STD at school, or was shot to death in a school shooting?
neither of these are desirable situations, yet BOTH have happened in our lifetime.
so, maybe we should be shielding our kids from both of these things, but more importantly, we should be showing them the consequences of their actions, and the right way to live.
I strongly believe that health classes (especially those that involve sex and STDs) should start at an earlier age, around 4th grade, when kids are starting to mature physically, and think they know what sex is.