The religion thread, predictibly, has brought out people who seem convinced that the opinion that life begins at conception is definitionally illegitimate because many of the people who hold it are religious. This seems flatly ridiculous to me.
The question of personhood is not definitionally religious, even if the only people interested in expanding society's definition of personhood are religious. Blacks are people, and those of us without any particular religious convictions are able to apprehend this, even if 150 years ago the only people much interested in prosecuting their claim to personhood were ministers and their flocks.
It is certainly possible to believe that life begins at conception without reference to God. And once a question is legitimately in the political sphere--in a way that, I would argue, the divinity of Christ or the Mohammedan succession is not--it's not really particularly reasonable to declare that people may not have reference to their own faith in deciding what they believe. Few people on the left seem worried by the fact that the anti-death-penalty movement gets much of its energy from left-wing churches, nor that those same churches have organized substantial opposition to the Iraq War.
Indeed, though I myself am pro-choice and mostly irreligious, it seems more likely to me that the main effect of faith is to spur people to embrace causes that are personally and socially inconvenient. Slaveowners didn't need religion to motivate them to defend slavery; they had a powerful financial interest in doing so. Similarly, the pro-choice movement, at least in my experience, gets most of its activist energy from reproductive-aged women who have a strong interest in being able to terminate an unwanted pregnancy.
By contrast, what self-interest was served by the abolitionist movement then, or the pro-life movement now? There's a legend among many pro-choicers that everyone in the pro-life movement is a patriarchal, selfish man who wants to force women to have babies in order to control them. In fact, women and men are roughly equally likely to be pro-life. The best that pro-lifers get out of their movement is--having to carry their own unwanted pregnancies to term.
Absent self-interest, you need some other motive, and Christianity provides a good one; the New Testament doesn't have much sympathy for the notion that you're too busy or too embarassed to follow your convictions. Obviously, there's also the social clustering of belief--Quakers tend to be environmentalists not necessarily because Jesus said so, but because the kind of people who are attracted to Quakerism are also attracted to left-wing causes. Likewise, Southern Baptists tend to vote Republicans for a number of reasons, of which religion may be the least.
I presume that no one, not even religion's most dogmatic opponents, believes that encouraging people to do what they think is right is a pernicious aspect of religion. Since observationally, almost none of them seem to think so when the religious person in question agrees with them, this seems like just another disingenuous way to attempt to shut down debate.
Now, that doesn't mean that religious arguments have a place in the public square. Opponents of gay marriage need a better reason than "God said no" to appeal to those of us who are skeptical that this God exists, or those who think that God said something else entirely. But it's not possible to remove religious motivations from politics, and it's far from clear to me that the country would be a better place if we had.