I hope this will be taken as helpful and not bashing in anyway. But we were just talking about this in my college class today. I am taking a Psychology class on Human S**ual Behavior. Since so many seem unsure and like they want more info I thought I would post a few things that were discussed. I have four kids of my own and have kinda followed this myself. But I admit I have not been as open as I probably should have been.
Anyhoooo a couple of interesting things to think about from my book, many from the SIECUS (its a family board, so I am going to talk clean, hard to do though and actually goes against what they reccomend
):
Less than 10% of teenagers list their parents as a major source of where they got information about the "topic".
Children from families that discussed the topic openly and comfortably are more likely to discuss the topic with their partners openly and comfortably once they become active.
School based education on the topic is very factual and value-free. Its the facts and just the facts. Parents discussing it can contextualize the information into the fabric of family values, ethics, morals and religious beliefs that school are unable to do.
Parents should not expect to have one big "Birds and the Bees Talk" and be done with the topic forever.
It is easiest to teach children the proper names of their private parts at the same time you teach them the names of all their other body parts. Yes, this means Preschoolers.
Next, experts reccomend that parents teach children about puberty and the changes it brings before puberty hits, at ages 6-9. These changes can be frightening to children if they don't know they are coming.
After puberty starts, consider explaining to girls about the changes that occur to boys in puberty and vice versa. Men should be able to relate to women with empathy and understanding and women to men. Hygiene skills should be emphasized at this time as well.
In upper elementary school it is good to start talking about the mechanics of reproduction in a positive way. Pregnancy and other consequences should also be discussed. And that physcial appearance does not determine a person's value, as well as communication and assertivenesss skills. If you are teaching your child abstinence, then you want to teach them how to communicate this belief.
Junior High Schoolers could have discussions about desires and feelings they might be having, and that love and s*x are not the same thing. Peer pressure, the right to say no, respecting limits set by a partner, exploitive relationships, healthy bodies (self-exams), and family values and morals should all be discussed repeatedly and in depth. Role-playing and other skills should be taught.
High Schoolers should continue to have discussions about accepting all types of people, that everyone has these kinds of feelings, (in whatever way is appropriate for your family values and morals), that love and s*x are not the same thing, that long term commitments take a lot of work (parenting and marriage) and are not magical things that "live happily ever after". And more and more discussions about how to communicate their beliefs to their bf/gf, how to respect a partners limits, self esteem, and if it works with your beliefs ways to prevent STD's and pregnancy.
I was laughing and said to him "well there ARE a few guys that know how they get there!" It was funny. AAHHH, do I REALLY have to take his innocents away!! 
