As many of you know, I'm a strong supporter of the seamless garment of life. I came to this position through my religion, but now I see a totally secular, anti-eugenics argument for such a position.
First, when does life begin? SG argument is at conception, but they don't tell you why. Genetics, however, DOES tell us why- for any bisexual species, the female egg and the male sperm have RNA only- half the chromosomes needed for the species. It's the second they come together, combine than RNA then start duplicating the resulting DNA, that the "blueprints" for that individual are created. If you're going to separate out individual lifespans at all, then conception is the logical choice.
Second, even secular humanists believe that taking a human life is wrong in some instances; aside from the obvious duty of government to protect society (an exception even the Roman Catholic Church taught until the mid 1980s). I argue therefore that technology has accomplished today what was only a dream in previous centuries- the ability to construct, out of welded steel, an escape-proof cell with only a food slot in the top, a drain in the bottom, and a shower that always runs. With such a cell, a far more horrific "life sentence" awaits the person who endangers human life or society- one where the person can be kept on display and shown to school children as a deterrent, for far cheaper than the endless appeals that go with a death penalty verdict.
Third, unjust war. War is sometimes a requirement- but even a secular humanist can see Augustine's logic of war of defense only. OK, maybe not the weaponry part- but certainly the "no revenge, no invasion" part.
Fourth Euthanasia- there is value in all human life, even suffering human life can teach us compassion. There's a reason why many disabled people end up heroes in their own communities.
Fifth, Eugenics- this being the real basis behind wanting to keep abortion legal, the hope that one day we will be able to filter children to be born who are "perfect" to some degree. This, to me, is one of those places where the knowledge of science outstrips the wisdom- where what is possible, and what should be, are different. Putting aside for the moment that we can't read the human genome enough to predict anything at all about how a fetus in the womb will turn out. Mutations, evolution teaches us, have value in helping the species survive. Do we really want to trust that natural method to imperfect human beings to judge who should, and who should not, survive?
Sixth, Economics- they claim the world is already overpopulated. Well, in reality, we raised enough food to feed 7.5 billion people last year- but about 1/7th of that rotted away unable to find a buyer. This tells me that the final lynch pin shouldn't be killing off the next generation- it should be using what we've learned in computer science about resource allocation to make sure even the poorest societies can survive. And that includes the million children a year killed in the United States because "their parents can't take care of them".
So there you have it- six completely secular reasons, without appeal to faith, to be pro-life; and not just Republican "protect human life between conception and natural birth" pro-life, but Catholic "protect human life from conception until natural death" pro-life.