To make sex is not complicate. Simply do what you like. The elementary rules of how to use a condom are complicate? I think, to learn reading is more complicate.
It's not to say that there is nothing to teach in sex. There are a lot of things to learn: How to give the partner more pleasure, different techniques, foreplay, problems with erection and orgasm and so on. But nobody (instead of the pedophiles) is teaching these things to children.
But, what people have in mind saying it's too complicate is another thing: These are the usual restrictions against sexuality in our society.
Something like sex is good only for procreation, only in a dark room, only with your wife. Or, in a more progressive variant, only if you have an equal power position, really love each other and so on. The child has to learn the various restrictions different people try to establish for sex and to choose and own set of restrictions, until he/she is "mature enough" to make sex. A really complicate thing.
The analogy fails to show the great danger of sex which has to be compared with an accident with a car. The real dangerous things related with sex - pregnancy and veneral disease - may be handled by allowing the children access to condoms and other contraceptives.
An interesting question we obtain if we ask for the analogon of the driving schools and driving tests (provided by experienced drivers).
Another question is that we allow children much earlier the participation in road traffic as the participation in sex - as pedestrians, later with a bicycle. What about analoguous sexual activities (caressing, exhibition, masturbation)?
Beginning at very early age, we start to teach the children the traffic rules. What about analogical teaching of rules for sexual behaviour? At least partially, this is forbidden by laws