Well, that is an interesting perspective, but it seems to me that there's something important left out of it. It seems to me that since the baby is dependent upon the mother to survive up to birth, that it's essentially engaged in a parasitic relationship with it's host.
The problem is that, whether it's fair to the life going on inside of the mother, it's still their body, and unlike in the case of suicide or mutilation, there are clear ways in which pregnancy can harm the mother's body more than an abotion. It can result in calcium deficiencies, pain, inability to work, and more. In extreme cases, labor can even result in death. The fact is, I think that inherent risks of the birth process balance things out. If pregnancy had absolutely no negative effects on a woman's body, your perspective might be justified, but that's not the case. It's not the same as killing a person in cold blood, considering the parasitic element and health risks carrying a baby involve. The baby is, albeit unconsciously, doing biological harm that it can't necessarily be held blameless for if we're going to regard it as a life.
The fact that it's a "natural process" doesn't mean we have no right to interfere. We already interfere with several natural processes that don't directly affect ourselves (such as killing animals for food, medicine, agriculture, etc), but somehow the ones that do directly affect ourselves are the ones you're going to consider too "sacred" to touch? That strikes me as kind of self-centered and hypocritical, if not ironic. We have the right to decide what other species live and die, but not the life of something that's living your own body? Seriously? Illness and death from illness are also a natural process, and yet I don't see any of you jumping to oppose the use of medicine.
If you adopt a moral system that forbids abortion, by all means don't get one. But... I don't think you have any right to make that decision for others.
I don't think every other civilization that forbade them was wrong. I just don't think forbidding abotion is in keeping with our countries ideals, which we're developing new understandings of all the time (for instance, abolishing slavery and giving women the right to vote). For countries founded on different principles, perhaps it is in keeping with their spirit.
Ultimately, my reason for wanting it legal is because I don't think the government should be involved in it, just like I don't want them banning alcohol. Essentially, I believe that we should limit the government's involvement in what people can and can't buy, including abotions. Granted, I don't think the Government should be paying for abotions or creating free abortion clinics either, but it should be legal. It's one thing if you want to protest against and create a social stigma around it, but to create a law regarding it would be unjust, since to do so clearly forces a particular understanding of an issue on which there can be reasonable disagreement upon people unnecessarily. In a religious community, you could reasonably create a stigma around abortion that's as much of a deterrent as any federal law. Shame and upbringing are powerful on their own.
One of the things that's good about our country is that people who have different religious beliefs and conceptions of morality are more free here to live according to their beliefs here than anywhere else. I don't feel that outlawing abotion is in keeping with that. You could choose to put it on the same level as killing people, but there's a huge difference if only in practical terms, because it's not necessary to outlaw abotion to maintain order. Societies have managed to maintain order even when exposing weak children to the elements was considered acceptable... this is still far less brutal than that practice.