When I lived in Germany I attended grades 4,5 and 6. In grade 5 or 6 (I forget which now), we had a bit of sex-ed in our biology class, for a while.
This was mainly the teaching of sex from a biological point of view, rather than a social (quite the opposite of what is done in North America, from what I gather). It was mostly fairly boring, to be honest!
We were taught how the organs work, what happens during puberty, why, what we can expect, but most of it was just biological mumbo jumbo (well, at the time, that's what it appeared to be).
During the last class we were shown a video of a man and a woman having sex. Nothing was censored. It wasn't porn - it was a video specifically produced with the intent of showing it in a sex-ed class. There was nothing too explicit.. I think we saw butts and boobs.. Everything else was hidden from view - but it was a bit more explicit than a softcore porn.
The social aspect of sex should be taught in sociology class. The biological aspect of it is far more important.