It boils down to preparedness. If a parent is prepared for sex education with their child, they will be more comfortable and open discussing it, and that will be rewarded with the child paying better attention to their parents when it comes to sex.
Keep information that you feed your child accurate and age appropriate. For instance, don't tell your child that babies are delivered by the stork; inform them accurate information such as "Babies come from mommy." That way, the child has the truth in their arsenal. However, you need to make sure that you keep it age appropriate. For instance, you wouldn't explain to an eight-year-old all the intimacies about sex; you would hold that off to say age twelve or thirteen when you would begin informing your child about protecting themselves in sexual situations.
Look for opportunities to talk your child about sex. As brought up on one of the websites I researched on, say if your eight-year-old child's friend's mother is pregnant, it would be a good opportunity to inject "Your friend's mommy is pregnant with a little brother or sister." Then allow your child to ask questions, it opens up conversation, breaks the ice if you will, to avenues of solid education between you and your child.
Now, early on in childhood is the time your child will be learning about the human body and where babies come from. Slowly, as your child ages and develops they will begin looking at the other sex. Now with girls this step is easier. Due to a woman's menstruation cycle it provides a golden opportunity to sit down with your daughter and begin discussing the later parts of sex education. Subjects such as relationships, sex, and protecting yourself need to become common topics. Now, don't overlook a boys need for this information as well, even though it will be harder to approach it needs to be done. Its been shown throughout many studies over the years that girls are informed about sex and relationships overwhelmingly more than boys and its because boys are often overlooked or parents never take on the difficult hurdle of discussing these topics and approaching their developing sons with these kind of conversations.