Hi M.,
When my son was 2 he asked where babies 'came out'. That was the day his sex education began. I decided right then and there to be very matter of fact about things--not to act embarrassed or uncomfortable (even when I was!). My parents were very cool about it and it helped me tremendously. I wanted our son to have a healthy view of sexuality. NOT the one commonly presented in our culture. One that viewed sex as being a wonderful gift from God for a husband & wife to share exclusively with each other & that comes with the huge blessing of procreation. (We have a Christian worldview too.)
Anyway, I just told him that day that mommies had a special hole different from the ones girls use at potty time, and that's where the baby comes out. Of course he wanted to see, as did my nephew when my SIL had the same talk w/her son. I told him that our naked bodies are great just like they are but that we do not share them with everyone. We talked about how it would be silly for Mommy to run down the street without clothes on. He seemed satisfied, so that was the end of that for the time being.
A year or two later, when he inquired, we talked about the differences between girl and boy parts. I stuck to the very basic functionality of our parts, i.e. girls have special parts for growing babies & boys for helping to make them. Again, he was easily satisfied. In this way we continued over the years, adding info to what he already knew. My policy was to tell him matter-of-factly enough to satisfy his curiosity & the level of knowledge he needed at that age and no more.
Now, at 12, he knows all the facts about sex and the last real frontier, I think we have is for my husband to talk to him about the changes that hormones are bringing about in his body & how to deal with them. He is getting ready to do that pretty soon.
Anyway, the result is that now, as a pre-teen, he knows the one man/one woman in marriage concept & accepts it. We say that sex in marriage is really beautiful & good & FUN, but it can also be like a fire. In the fireplace where it belongs, it's warm & good. Out of the fire place, it can burn down the whole house. In this way we try to cover the cons of permiscuous behavior without turning his whole concept of sex into a taboo.
We also have the luxury of teaching him at home, so he is relatively shielded from those discussions with other kids. However, as a child, I was not, even in private Christian school, so I'm really sympathetic with your situation. I was really glad that my parents told me most of what I needed to know from a Biblical viewpoint before the world had its say. It helped me to hold on to what I knew was right, especially when I got into my teens.
Hope that helps!
B.