How to talk about sex-education to your child?

We don't talk about sex with the children especially with the girl. But what we do is to give them a hint that they needed our advice. Like when watching tv or a movie with a scene where we can inject the idea, we do it. And we always emphasize that boys should ask the father and girls should ask the mother. In other words, sex is a men's talk for the boys since it would be awkward for the boy to ask his mother about it.