It's totally naive on the part of grown-ups to think that kids would have no knowledge of birth-control pills or condoms, if we don't teach them about them in school. Come on, already, every other commercial break in every TV show teens watch have commercials for them. Kids aren't anymore stupid today than they were 30 years ago, when I was a kid and I knew all about birth control and condoms.
I think the key is to just keep the lines of communication open. If as parents, we talk early and often about our sexual values with our kids, and encourage and exhort them to always come to us with any questions or problems they have, then most issues can be dealt with. If the child is old enough to ask the question, they are old enough to get an age-appropriate answer. I've never yet told either of my kid's, "Umm, you're too little to know about that." They are hammered out there with sexual messages all the time, even when we think we are shielding them from it, we aren't. Rather than try to be a shield, it's better to be a filter. Filter the messages out there, messages from it's important to be "sexy" or "hot", to it's OK to kiss a girl and like it, with God's perspective. If we don't explain things in an age-appropriate manner, they will learn to look elsewhere for answers, and there are plenty of things out there from which they will start drawing conclusions about.
I do talk with our kids, especially our daughter, about sexual things. We talk about everything from how entering into casual sexual encounters can have a horrible impact on developing a healthy life-long relationship with a spouse, how birth control pills do nothing to protect from STD's, how one can indeed get pregnant on one's very first time, etc., etc., etc.. Sometimes my daughter rolls her eyes and goes ewww, other times she asks very serious questions, which I always answer, no matter how much I'd love to shield her from the subject. The reason why I do this is because my parents would have died rather than discuss sex with my sisters and brother and I. The result was that I was sexually abused and didn't talk with them about it, and my sister's and brother were married at 17, 18 and 18. I really wished they would have been more open to discussion, but back then parents really didn't talk about sex that much with kids, and when they did it was much more about the 'mechanics' of sex, rather than the values. I do remember the "Only when you're married" bit, but no reasons were given as to why. God was good, and in spite of the odds, my siblings are all still married to the original spouses and I've been healed from the sexual abuse, but I think life could have been better had we felt we could discuss things with mom and dad. But, when every question asked was answered with "You're too little..." after awhile, we simply stopped asking them.