This is a hard issue with no perfect solution. I don't like the idea of my teen daughter getting Plan B without me knowing about it. Side effects are rare but still....
Suppose she starts bleeding or passes out in the middle of the night, and I don't even know what to tell the paramedics? Then I think about all the side effects of unprotected sex besides pregnancy--AIDS, herpes, other STD's. Yikes. I want my teen baby girl to be zipped into a full-body condom until age 25.
Most young women who think they are pregnant will talk to a responsible adult, usually a parent. Ideally they will go over their options and make a decision about how to proceed: many people decide to continue with the pregnancy and try to raise the child, regardless of the impact on the girl and her future. Others (not as many as in the past, where authorities routinely took the baby from the girl's arms and she never saw it again) try to arrange for an adoption. And some will decide to have an abortion. There are no other possibilities.
A pregnancy is a big deal. It is more risky to a woman's health than any birth control method, and is the most life-threatening health issue most women face. Having a baby is also a big deal, and it is a lot to expect that a teenaged girl can realistically evaluate a 20+ year personal, legal, financial and emotional committment to another human being.
Most of us would agree that most 15 or 16 years olds are not mature enough to sign a rental lease, or to get married. But they are supposed to be able to take on the awesome responsibility of producing a functioning human being? Of course, in most of the situations that I personally know of, it is the grandparents (mainly grandmother) and other relatives who end up raising the baby. Sometime it works out fine and sometimes not.
I am radically pro-choice because I value babies and women's lives more than I value unformed masses of cells. But I honestly don't know how I would feel if my own daughter took Plan B without me knowing.