Wittgenstein argues in the Lecture on Ethics that all attempts to talk ethics or religion result in nonsense. Roughly speaking, this kind of discourse is not rational, as he sees it, and this suggests that attempts to reason about ethics might be misguided. I think there's something to this.
Reading Gary Gutting's (rather good) piece on abortion and personhood in The Stone reminded me that most people's thinking (though not necessarily their feeling) about abortion seems off to me. Standard ways of conceptualizing the issue fail to capture standard intuitions, that is. Gutting says: "The above analysis does not settle the vexed question of abortion. That would require answering the question of what makes a person a person." But I'm not convinced this is true. I really don't think the concept of a person helps at all in making sense of our intuitions or feelings here.
As I see it, the basic 'pro-life' idea is that life is a miracle worthy of respect. The appropriate response is awe. Contraception, abortion, suicide, and euthanasia are all, in their different ways, forms of 'miracle control,' and thus both absurd and blasphemous. One does not rationally manage a miraculous gift. One accepts it as gratefully as one can, and cherishes it. Nothing else is an appropriate response.
What about abortion in cases of rape or incest? Here the idea of a gift or miracle makes much less sense. It isn't, perhaps, impossible to experience these pregnancies as a gift, but it is surely rare. Whether doing so is saintly or just weird is not for me to say. But it is at least understandable for them to be thought of as part of the rape or incest itself, part of that terrible episode, and thus as not at all worthy of respect. After a baby has been born a new episode has begun, and infanticide would be wholly inappropriate, but before then abortion might well be the least bad option. The same kind of thing might be said about abortion in cases of extreme disability. It might be more natural to experience pregnancy in cases where the child, if born, will suffer terribly as mistakes rather than as gifts. And so, again, abortion might be appropriate. This, I think, is how most pro-life people feel. (Assuming they are sincere and not just vengeful puritans, unthinking woman-haters, etc.)
A problem with this view (it can hardly be called a theory) is that it is so vague and subjective. The very idea of appropriateness is almost hopelessly vague. And how could we ever pin down what the natural or appropriate way to experience something is? Or what the appropriate response to a miracle is? I don't think we can. Which means there are serious limits to what philosophy can contribute to debates about the merits of the pro-life position. Perhaps the best it can do is to point out when theories distort the real issues.
What about the 'pro-choice' view? I think it's probably similarly vague. Where it diverges from the pro-life view (if it does at all--after all, the pro-life view that I have described is quite capable of accepting some abortions, and this might qualify it as pro-choice on some conceptions of that view) is in bringing in concern for the pregnant woman too. Her life is worthy of respect and might be seriously disrupted by an unwanted pregnancy. The most appropriate way to respect her might be to end the pregnancy. Perhaps abortion is always bad, but it could be the least bad option even in cases where normal pro-life people would not support it. In short, I pretty much agree with Ronald Dworkin's take on the subject. (Judith Jarvis Thomson argues plausibly that he gets the pro-life position wrong here, but the evidence cited by Gutting suggests to me that Dworkin is actually right about what most people (not the Pope) believe.)
The bottom line: many of the ethical questions that most concern us (gay marriage is another one) turn on appropriate ways to respect things we value (life, autonomy, love, etc.). Reason is little help here. As the Greeks found out, one way people respect the dead is by burying their bodies, but another is by burning them. Reason will not tell us which is better. And metaphysical questions about the essence of persons or of marriage, for instance, are red herrings. Try as we might, we cannot turn ethical questions into technical (or quasi-scientific, metaphysical) ones.