Do you know what the content of sex ed when taught to younger classes is, and why the curriculum was adopted?
Because, the core of it was to ensure that said 3rd graders knew two things: 1. What the correct anatomical names for their genitals - parts of their bodies - are, and 2. What is appropriate and inappropriate touching.
Now, the reason you want to teach these things to children, really as young as possible, is because it turns out there's been quite a lot of child abuse cases where the defense case has rested on the fact that the victims don't actually know the anatomical names of body parts. And as a result when you go to prosecute, you can't conclusively prove from their testimony what was taking place, which it turns out has a habit of not convincing juries.
A child who only will sort of gesture to the lower part of their body, because they don't know to volunteer the correct name of the body part, again, it turns out, leads to a court system which fails to apply justice properly, and also increases the general risk of abusers being able to operate because they convince their victims that everything that's happening is "okay".
Oh, and as it turns out, it's also very hard to teach children how to identify this sort of thing without touching on the idea of their being multiple sexualities, because of course to teach something like appropriate and inappropriate touching you are quite likely (a) having to service questions from children who'll ask why it's not inappropriate for mommy and daddy, or (b) have to ask what about daddy and daddy or mommy and mommy and (c) who might struggle with the notion if it's not correctly identified that the inappropriate thing will apply regardless of gender, and yet can also be appropriate for adults regardless of gender.
There's a reason no one targeting primary school level physical education classes wants to say anything more then "sex ed" because, if they had to actually cover the content of what they were opposing, they'd rather look they were basically defending child predators. Which, though not directly, they are.