Geek points to anyone who got the Monty Python reference.
Over the holidays I reading Denial of the Soul by M. Scott Peck. It's a philosophical and religious look at the ethics of euthanasia. It dips a little into the abortion and capital punishment debates as examples of how our society views willfully taking life.
Peck complains bitterly about not being able to understand how pro-life supporters can also support capital punishment (as is statistically likely). He also can't understand how pro-choice supporters tend to lobby against capital punishment. In his view, the pro-lifers hold that all life is sacred, therefore they should be as against killing criminals as they are about killing fetuses. And pro-choicers claim killing is acceptable in one circumstance so why should they be against it in another?
Aside from the shakiness of his explanations, I think he's missed a rather important note. It's not about life and the relative sacredness of it at all.
Pro-life/pro capital-punishment supporters tend to come from a conservative, religious background. Their views become consistent if you see it as about punishment rather than life. A murderer or rapist deserves the ultimate punishment in their books, having their own lives taken. A girl who has had sex should not be able to scamper off consequence free, she should have to bear her God-given punishment of sacrificing nine months of her life to carry a baby. This also ties in to the usual pro-life exceptions of rape and incest. Those were not chosen acts of sin and thus no punishment is necessary.
Pro-choice/anti capital-punishment supporters statistically tend to be liberal and somewhat agnostic. For them it becomes more about the right of the government or another minority to impose their will on the individual. The state does not have the right to choose life or death for any of its citizens be they fetuses or mass-murderers.
Personally, I think the situation is traumatic and difficult enough that no formula can ever be sufficient. Capital punishment can't be applied fairly and thus shouldn't be applied at all. Abortion is a horrific decision which belongs to the individual and I think the psychic scars are more than sufficient "punishment" if any punishment is necessary.
To paraphrase Gandalf: Some who die deserve life. Until you can dispense both equally, have a care to what you choose.