Talk with them about it. Discuss the values that you wish to impart -- maybe things like:
This is what people do when they are old enough to handle a really close relationship. It's something I encourage you to wait until you're a real adult to do, with the right person, at the right time. Never feel pressured--or pressure. Come talk to me if you have any questions about anything at all. (And if there's some sort of relationship in the book that you think is bad, e.g., rape, irresponsible sex, harmful sex, etc.) then you can bring that up.
it's okay. It's your job.

And it's the job of your kid to be mortified to talk with you... but you're breaking down barriers, so that's good!
If you're interested, check out a Seattle-based family educator who specializes on talking about the birds and the bees with kids of all ages--as appropriate for their age. You could even email her your question. She's very good!
http://www.birdsandbeesandkids.com/