"Of course there are consequences of sex that children need to be protected from: Sexually transmitted diseases, pregnancy, physical damage, hurt feelings, social condemnation."

True.
The first three can be protected relatively easily. The last of the five is more complex matter.

"When you are riding a bicycle obey traffic laws and avoid busy streets, if using hand brakes always apply the brake to the rear wheel first, always wear good shoes because if the brakes fail you will need to use your feet; never point the rifle at anything that you mind getting shot, keep your finger off the trigger until you are ready to shoot, keep your hands off the rifle if anyone is down range, every weapon is loaded even if it's not; never swim without a buddy, never go swimming in an area that hasn't been checked to be safe, NEVER dive into water if you aren't SURE about the depth, beware of rip currents, beware of moving water, don't go swimming in polluted waters."

Successful and otherwise physically safe use of bicycle or swimming rarely causes social condemnation.

Sex is a different matter... and so is use of firearms in a lot of world.

"But with sex - an activity that is absolutely essential to human survival - we simply tell children: don't do it, don't even think about it until you're an adult - at which point you will magically become competent.

That ... is insane."

Indeed insane.
While children are often told not to play with firearms, where this applies they do not magically become competent at a chronological age, but have a lot of hoops to jump through, or may never qualify for a licence, even when an adult.

Sex, unlike use of firearms or automobiles, is essential for human survival. Since it is a high stakes, dangerous activity, people ought to be taught over long period of time, whether before or after chronological age of adulthood, but either case well before they actually practice sex. But since sex is essential, people ought to become qualified by a reasonable age.