Actually, as a parent of 3 children 4, 8 and 10... we openly discuss sex and sexual differences. I have bought several age appropriate books that discuss and show pictures about sexuality and what sexual intercourse is and how babies are made etc. We have read through them, they are readily available for them to pick up and read any time they want, and any question asked of me is answered straight up in a matter of fact manner. My children will not grow up thinking sex is a taboo subject, and an inappropriate behaviour the way I was and will have an absolute clear understanding of what it is and the reality and ramifications of their actions.
Finally, in my opinion, onset of puberty indicates onset of sexuality.... and lets face it... one is the horniest when they are a teenager with those super raging hormones. So I will assume that my children will engage or want to engage in sexual intercourse in their teens, as that is what their bodies are urging them to do. So, as their parent, it is my responsibility to arm them with knowledge and tools (birthcontrol, condoms, etc...) so that they can be safe and healthy. All the information I present to my kids about sexuality is not done in a salacious manner, but rather in a matter of fact way that presents human sexuality as normal and healthy. That attitude is what I wish for my children.
In Holland, sex education is an ongoing part of the curriculum of children from primary through to highschool, and as such they have one of the lowest rates of teenage pregnancy in the world. Unlike the US, which actively restricts sex education, instead opting for abstinence education.... which given their teen pregnancy rate is not working AT ALL. So personally, I opt for knowledge.... the best tool in the arsenal for healthy, safe and preventative sexual activity.