I just wrote a hub about this very thing this morning. I firmly believe that a teenager having sex should have the common sense to get the pill.
Frankly, if they aren't asking for it, and they are having sex, they aren't being responsible. I do not think that adults should have to consent to this. I don't condone sex at a young age, but teenagers are going to do whatever they want, regardless of the rules and restrictions you put on them.
Honestly, if you tell them to do something, they will find a way around it. Most of the time, no amount of talking or rules are going to get through to teenagers. They are going to be having sex, its just the way this day in age is - so unfortunately, its better to be safe than sorry. The pill is the best option.
Besides, the pill is also used for other reasons, such as regulating the period and some forms of horomone imbalances in young girls, as well as acne. Its not always birth control - but if a teen girl is having sex, definitely, she should be on the pill.
You can't stop the sex, kids learn the hard way, I know I did - but you can stop the pregnancy.