As far as the scenario, that's our DD. Her friends come from very religious backgrounds. The school system bewilderingly doesn't address this until 8th grade... by which time many children who were held back for various reasons (and are thus more at risk to start with) are already engaged in sexual activities. And even when the school system finally does get around to it, it's abstinence only, which has been well documented to lead to exactly the same outcome we're currently experiencing... as of 2010, LA had the 5th highest teen pregnancy rate in the nation. All of our neighboring states, who do it mostly the same way, outrank us.
So yeah, it's pretty much all on DW and I. DW assumed the lead.
We handled it in phases. Phase I was discussion of the biological differences in boys and girls, and the physical maturation processes. DW provided American Girls books, added her own lectures, and took DD to the store to acquire some preparatory items.
Phase II was discussion of the biological process of human reproduction, contraception, and disease prevention. Even though DW was very uncomfortable with this (and I much less so), she decided she was the one best qualified to discuss this, so she prepared some materials and gave a morning over to teaching DW on this topic while I was away at work, much like it was another homeschool science lesson.
Since then, the door has officially been opened on the topic of human sexuality, and we address any of DD's questions whenever or however they come up.
We set ourselves some firm time limits to have these discussions, otherwise it'd be easy to keep putting them off, because they're uncomfortable. Since DW had her first menses at 9, Phase I had to be done before DD turned 9. For Phase II, we reached into our own memories and decided we'd had sex ed in 5th grade, and with DD accelerated into 5th grade, we decided to do it after she turned 10. DD had been 10 for several months, and I'd prodded DW a few times, before it finally got taken care of.
Side benefit: We've officially upgraded DD's media access to our definition of PG-13, and now DD requests to watch reruns of Friends, DW's favorite show of all time.