I can’t believe I forgot to post this here at the time!
I wrote a post for The F Word titled How do you teach a child about sex? discussing the ways in which I am doing sex education with my young son and asking readers how they’re doing the same. So far the conversation there has been great, and I’d love to hear your input too. 
In parenting/caring for kids, the subject of sex seems to best be handled piece-by-piece like just about every subject (including the life cycle of flowers, chemistry, math, cooking) but more proactive work is needed since our culture has horrid ideas about sex (esp. misogynistic, homophobic, and transphobic). I recently wrote a little piece about our day and I thought things went pretty well.
I think it’s good to start by teaching the proper names for anatomy and go from there. Be honest in response to questions a child might have about his/her body, or anything else related to sex. when they were younger we answered their questions directly, but kept it to a minimum. As they’ve gotten older we’ve tried to open up a dialogue where we do go into detail about sex. We also try to de-mystify sex with education so they have accurate info rather than getting old wives tales. Our Bodies, Ourselves regularly leaves the bookcase, and that makes me a little less worried
I don’t think it’s possible to give TMI when it comes to sex. I can handle an awkward conversation (or 10, lol), but I can’t handle a teenage girl out there learning by experience.
These conversations really need to happen on a regular basis in order to be effective. They don’t have to be big, serious talks..We try to casually bring up sex topics. For example, I told the girls about a friend of mine who got pregnant in high school and how difficult it was for her. That was a good opener to talk about what guys are like, if they know of girls their age having sex, etc. I also use that time to let the girls know that they can tell us when they need birth control without being in trouble or judgment.
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If we are frank and at ease with sex talks I think there is a better than average chance that the girls will feel like they can come to us when they are thinking about doing it. And we always try to bring self-respect up, and we give them the language to teach men (and everyone, really) how they want to be treated.