@Thefartingsofaofdenmarkstreet
I'm not saying that it's ok for children to be having sex at primary school. I'm saying it's not ok to pretend to children that sex doesn't exist and not to educate them about it until an arbitrary age when adults decide they are old enough to handle it. This is because if a child is having sexual feelings when they are in primary school, if they've not received any education about it, what are they going to do with those feelings? How might those feelings be expressed? Who might they end up hurting when they try and express those feelings? Sex education is important for this reason.
Buy asexuality is an absence of sexual feelings, so why would kids need to know about it anyway? Surely you don't really need to even know about asexuality until, well, adulthood really when you might start wondering why you have never fancied anyone? Certainly not at primary school where is completely normal not to have any sexual feelings? 
I'm confused as to why you're confused by this.
We teach children about different sexual orientations as part of helping them to understand the world around us and how different people live their lives.
As such, at school, we teach children that some people have two mummies or two daddies, because not everyone is heterosexual.
As part of that, we can also teach them that some people never have any sexual feelings at all, and that's ok.
It's simply part of educating them about how the world works and how different people fit into the world.
It also means as they grow up and start to experience (or not experience, in my case) sexual feelings, they can understand and interpret their feelings within the frameworks of sexuality they have learned about.
Again, if you replace asexuality with homosexuality in what you've written, hopefully you can understand why what you've said is troublesome. If we didn't teach kids about homosexuality, people who are homosexual would just have to 'figure it out' when they got to adulthood. By then they might already have experienced a lot of pain and trauma in trying to fit themselves into a society where they didn't feel they belonged. Same for asexual people. Knowing it exists means that, as you grow up, if you don't start to experience sexual feelings, you'll know that's normal and ok too. Rather than thinking you're a freak of nature.
I hope that explains things for you.