Just take the kids camping, then everybody enjoy a little skinny dipping. And when questions arise, just answer them. Or bath time, little Bobby and little Suzie won't have any questions very early on, but when they do, straightforward, honest answers given without embarrassment will do the trick.
Do not get me wrong, I don't dismiss the importance of sex education. But first and foremost, I sincerely belief a real lack of shame about sexual matters is the healthiest thing you can ever give your kids.