My maternal grandmother never explained sex to my mother. My mother fell pregnant at eighteen because (by her own confession) she was very niave about sex and didn't really know about contraception.
Like her mother before her, my mother never mentioned anything to do with sex to me either! In fact she was quite the prude really - maybe because of her own upbringing.

Instead I had really good, interesting and gentle introduction to sex education as a child which was in no way disturbing to either me or my peers. Other people finding the notion of children drawing gentle pictures of mummies and daddies cuddling in bed disturbing, says more to me about their own feelings about sex being a 'taboo' matter, than anything. Of course, because I was actually there, I don't have to imagine how dreadful it might have been like. I know they were lovely lessons nd not at all weird or wrong, and the teaching of the subject matter was done with warmth, gentleness and sensitivity.

Despite a complete absence of parental discussions about sex, I knew enough about sex and pregnancy from school, that by the time I reached a point where I became sexually active, I was informed enough not to fall pregnant. I'm more than glad the school system saw fit to arm me with the essential information that my parents did not discuss!
Without schools sex education all I would have had would have been schoolground rumour.

Not all parents want to talk about sex, and no amount of arguing that they SHOULD be doing so, will alter the fact that many DON'T and WON'T.
I believe that schools have a responsibility to young people, to ENSURE that they recieve essential information in an open and healthy fashion.