I just wanted to thank Mummie and the other posters for this thread. Once Mummie got involved and was able to eloquently express her differing views, it resulted in some really interesting debate. My children are still very small, 3 and 16 months. But my daughter is facinated/obsessed by the notions of families. Everything is turned into a family. She also worries about many of the children in her books if they don't have mummies. She wants to know where their mummies are.
I am not sure who posted it, but this thread has got me thinking about how I can introduce to her the notion of familes with two mummies, or two daddies. Doing this, for me, isn't about sex education. It is about understanding from the beginning that there are ways that people live which aren't the same as her life, and those are just as normal as what is normal for her.
Personally, even if it is labelled sex education, my opinion is that for most of junior school the point of it is broadening childrens understanding of how the world works, and so that even if it is something that they have not personally come into contact with, they can understand how it works and accept it.
I think that the aim of introducing children to experiences and situations beyond their own experience at a very early age is so that they understand that all variations on families and relationships are fine.
With regards to 10 year olds, I don't think that all 10 year olds will be ready for sex education, but I'm sure there are some that are. Those that aren't may be even more worried by the uninformed information of those that are, than by some sensitively given fact. So I'd say that 10 isn't too young. For explaining some of the differences in relationship types, I'd say it is too old.