Personally I have already started talking to my wee girl about these issues, we read 'where willy went' regularly and 'mummy laid an egg'.
Both entirely age appropriate and with just enough information to stem her naturally and normal curiousity about it, but not too much to freak her out.

I have had other parents utterly horrified that I speak to her about this stuff, but I would rather she learns about stuff like this from me, and feels able to ask me about anything.

So to a certain extent I think there is a responsibility on parents to educate there children, in addition however I think that sex education is an essential part of a general education and really like Denzarks idea of it being part of a broader 'society class'.

The sex education I received at the all girls school I attended was dismal, a single day where we got to,pass around a weird wooden phallus, and then spent half the day shouting NO! and punching pillows. it left me prepared to identify mahogany dildo's, and to defend myself from random items of bedding should they attack.