Among other things I tend think that the reason it ain't taught is that so many of us differ on various issues concerning Sex education.
If the Government were to develop a curricular, I expect it would consist of such things as: Contraception, Condoms,etc some religious institutions would have problems with this being taught because they don't agree with them. Some lobby groups would also want topics on homosexuality being taught etc. Generally it would be hard to agree on what to teach and what not to. There r various other reasons also.

If I may ask, what would you like to know?

Disclaimer: You have to be over 18 years of age.

That would have been taught in high school?

I think you are looking at it from a philosophical point of view and rather broadly.
There is no universally accepted meaning or view of sex and can only be taught based a society that profess certain values.

I will therefore answer your question based on what I was taught.
My Faith teaches that Sex is supposed to be between a man and his wife ( they become man and wife only after wedding in church)
Actually the church ceremony is half a marriage and the couple perform the other half by meeting body to body and thus the marriage is whole. Sex has 2 purposes in a marriage being procreation and support of the couple by one another. The couple are supposed to be open to life meaning that there shall be no contraception and the only acceptable means of family planning is the natural method.

@Kaka,

How does sex foster between a couple?

So @ tycho made a typo error! pardoned.
Support is basically in terms of bodily needs (read pleasure)