Please remember how you felt when your mother brought up sex or boys' savage desires or why buy the cow when you get the milk for free or "the loving and mature act of a man and woman" (depending on the kind of mother you had). Remember how you felt when you caught a glimpse of your parents in the bedroom, in their underwear. Remember how you felt when your uncle Archie made those gross, leering remarks about your "development." The operative word here is eew. Your daughters feel the same way. I don't want to be a grandmother! When I first met her boyfriend, I wanted to strangle him.
They don't want to hear about how cool you were. They don't want to know the details of what you did or how you felt. (Let me be clear: They do not want to hear about your unusual anatomical details, they do not want to know your favorite sexual position, they do not want to know about the first time you masturbated, and they do not want to know what kind of orgasms you have.) They do not want to hear about you. They're teenagers; it's all about them all the time, and every time you bring up your own feelings and your own experience of sexuality, it's like a thick curtain falls in front of their eyes and ears. It says: ancient history.
But—you can talk about the wider world of sex in lots of detail without ever claiming those details as your own. You want to study up. If you slept during certain parts of biology and you have no idea where your cervix is or what it's for or you've never had contact with a clitoris (and aren't sure which syllable to emphasize), now's the time for some homework. You should be able to use the right words for the things you're going to be talking about, and if you're given to blushing, own it. You can say, Grandma never really talked to me about this stuff, so I'm a little awkward. Then you plow ahead. (Secretly, she may appreciate your perseverance, even as she's saying, Sheesh, Ma, you're beet red!) Make sure she really knows the facts (not just the health class lectures), and do your best to explode all those middle school myths, which are, believe it or not, just as prevalent as ever: You can't get pregnant if you don't come. You can't get pregnant if he doesn't ejaculate. You can't get pregnant if you have your period. And there are more, and more pernicious, myths that still rule: Boys won't like you if you don't. Boys will think you're cool if you don't make too big a deal out of sex. Getting drunk is sexy. I would just list all the stuff that girls tell each other, and boys tell girls, and I would talk it through, the same way—sensibly, good-naturedly, and with complete certainty—you explained to her that those diet pills advertised at 1 a.m. don't actually work and the miraculous breast enlargement cream doesn't either.