Hi anon, I am a parent but I wasn't worrying about teen pregnancy when he was that age!
Still, just make sure your kids get a good sexual education and understand properly the processes and possible consequences (I don't agree with you that pregnancy is inevitable when you are having sex). Make sure they understand about contraception and keep on giving them that message - including the boys if you have them as it takes two to tango! If your beliefs are that pre-marital sex is wrong, then you need to put that over to them, but also be realistic and understand that they may not share that belief, so make sure they are educated and prepared just in case. It is not terrible to buy your teenager a pack of condoms - better that they have safe sex than unsafe sex and you are naive if you think your beliefs will have that much influence over them. Make sure they are brought up to believe in themselves and not to just follow the crowd so that they are better able to resist peer pressure.
As Robyn said, it isn't as much of an issue as you might imagine from watching Oprah etc, it is still quite unusual. Most teenagers who get pregnant tend to be those with fewer positive other options open to them; they are not getting a good education, are not lining up for a good career, etc etc so they can see it as a good option and a way to join the adult world when other ways are not there for them. Make sure your kids have plenty of options and opportunities.
My son is 18 and a few of the kids who were at his school are having children already. One boy of 18 has just fathered his third child by three different girls. None of my son's close friends are in that situation though and they all think the ones who are, are completely barmy. They do just as much tut-tutting as we do about it. Why the difference? I'm not sure. His group of friends did no better at school or anything and many are unemployed (tough area) but they are all intelligent and have lots of interests and things going on in their lives, and a good network of friends. They all have loving families, whether with one or two parents, who have taken a genuine interest and actively raised their kids. The ones having babies are the ones who don't seem to have much else...