Assume I'm talking in specific about U.S. policy, but in general about ethics. While I don't believe that the U.S. policy ought to be instituted everywhere in the world, I do believe that the ethical questions and the correct answers to them are indeed universal. Of course, in a society that does not actually value human rights (and yes, the U.S. falls down here in more places than it should) many of the things I take as axioms would be controversial. Nonetheless, I stand by them.
This is not about the state. This is about society. The state's actions are the actions of society by proxy. The state's laws are the choices of society by proxy. When things are important, we put them into law so that they may be handled as fairly and consistently as possible. Because not every situation is the same, we entrust government officials with interpreting and implementing the law so that consistency does not override compassion.
In the case of children, the starting point is the idea that as a society, we consider parents to generally be "in charge" of their children. But at the same time, we know there are situations where this cannot be allowed. Children cannot be sold. They cannot be forced to toil in factories. They must be given at least a cursory education. They may be punished, but not abused. They must be cared for. Further, "parents" isn't quite right, because the people responsible for these things aren't necessarily the parents. They are the "legal guardians". That includes whoever cares for and makes decisions for a child, whether it's their biological parents, step-parents, adopted parents, grandparents, foster family, or whoever.
So, what is the philosophical foundation upon which this distinction rests, between the things that guardians are allowed to do to/for the children in their care and the things they are not?
My interpretation is that children must be treated as equal to adults. I don't mean that we don't or shouldn't recognize that they're not ready to exercise full responsibility for their actions—but even as we are allowing them to make mistakes or protecting them from making mistakes, they are still equal. It is very very easy to miss that, to treat them as less than a full human being just because their experience and emotional development are still a work in progress.
When the state steps in to a bad situation between a guardian and a child, it's doing so because of the law, because we as a society have decided that there are limits on what should be allowed. In general, these limits are quite broad, and guardians are usually given the benefit of the doubt when there is a doubt. (In fact, they're probably given the benefit of the doubt far more than the situation deserves.)
Why are there limits? Because as I said before, the guardians only hold the rights of their children in trust, because the guardians hold a position of great power in this relationship, and because we know for a fact that there are adults who abuse that position of power and authority, and have decided that children must be protected from such abuse. The vast majority of these protections are in place in an attempt to protect all minors, even those who we do not yet consider competent enough to exercise their judgement alone. They are put into the care of a different guardian who will hopefully do better.
We also have a principle where children can argue before the court that they deserve if not the full rights of an adult, at the least the autonomy of an adult. We do this because we recognize that some minors are far more mature than some adults. This gives a further release valve, by allowing for those minors who are demonstrably capable of making well-reasoned decisions to cease having a guardian altogether. There are generally limits on what the person, who is still a minor, is allowed to do, and government officials have the duty to monitor their well-being—but it's a much looser constraint than the guardian-child relationship. I think part of why this mechanism exists is that we know that once they reach a certain age, minors will run away from home to escape abusive situations. This mechanism is an attempt to replicate that in a more controlled way, in the hopes that at least some minors will choose this instead of choosing to run away, and that the support of social services will help them finish growing.
All of this is consistent with the idea that the minor's welfare matters, that they should be treated as a full human being even when we don't trust their judgement yet, and that parents are only the first most obvious trustee for the rights of a child.
I don't think the law, or most peoples' reflection on these things, are as explicit as I've laid them out here. But I do believe they come from a similar consistent sense of justice. It's a hard balance, because people are very very attached to their children—and because we know that even that attachment can sometimes cause problems. That's why this is public policy, and ethics, and not engineering.
And to connect this to our debate here: the tragedy of a minor becoming pregnant and disagreeing with her guardian about what course of action to take is tremendous, whether she's being forced into bearing a child or being forced into having an abortion. Not only does the mere fact that the minor is capable of becoming pregnant mean that she is at least coming near to the point where we consider her to be competent to make her own decisions, but any extended legal argument for removing guardianship, or emancipating the minor, or anything like that will take long enough to make the question at hand moot, as the pregnancy will likely have come to term.
Personally, if I had been in that situation as a teen I would be furious with my parents, and would take whatever further steps necessary to place myself in a more acceptable position after the pregnancy was over. If they would be willing to force my choice that way, I would not consider them fit to be my guardians. But then, I was exactly the kind of person who could reasonably argue for emancipation if I thought it necessary. Still, I expect I could have been forced to either bear a child or terminate a pregnancy by my parents before achieving emancipation if something like that happened, and that would have been awful.
The nastier side of things is for those who have strong opinions, but are not so sure of themselves. Who are worried about their ability to support themselves. Those who find the family bonds more important than the abuse being heaped upon them. They probably have no idea that anything like emancipation is possible—and even if it were, they probably wouldn't think to try to achieve it. If they're lucky, they might have other relatives they could go to for support who could contest the guardianship. Someone like this could also end up being forced into an action she doesn't want, but very well may end up having to remain with those who forced her into that action. That would be really really stressful.
Finally, note that I think the U.S. system already addresses these scenarios. It's just that I think it prioritizes the judgement of the guardian over that of the minor more than it should, and that in this specific scenario there's just not enough time to properly judge the maturity of the minor. (And, of course, a lot of social conservatives are arguing to roll some of these protections back.)
Overall, it's just really really hard to get right, because it's terribly hard to judge the content of peoples' hearts.
The real tragedy, of course, which we seem to have lost sight of, is simply that girls are made pregnant before they are ready to have a child. And again, the only solution to that is education and easy access to contraceptives for minors. It's a lot easier to avoid becoming pregnant if you know how—and we've all been young and know that for many young people, abstinence is a lot to ask. Hormones are pretty strong, huh? That's part of why we don't trust the judgement of minors.
But teens aren't universally stupid. They can in fact think about their situation and what they want to do. And the vast majority don't want to become parents as teenagers. Those who do, or think they do... well... trying to prevent someone from becoming pregnant if they really want to is a little tricky.
Still, the very least we can do is make sure that everyone knows how to choose not to. It's not perfect, but it's better than nothing.
It's very depressing that so many people in this country think that "nothing" is the right answer.