Religious conservatives, or at least the loud ones, reject the argument of a woman’s right to choose an abortion, because what is at stake isn’t merely choice, but also life. Yet many times, abortion is the only moral choice. Not because it's "a woman's right to choose," but because — as both a personal choice and a social imperative — human life comes through the quality of life, not merely the quantity. There is a natural continuum, from simpler to more complex life, and a look at some of our behavior shows that we seem to know the difference intuitively.
At one end of the continuum are bacteria, which were the only form of life on earth for two billion years. They can, in ideal conditions reproduce — double their numbers — every fifteen to twenty minutes. Their growth is exponential. Taking the more conservative reproduction time of twenty minutes, this means that, under ideal conditions, one bacterium can produce 1072 bacteria in 24 hours. For comparison, some scientists have estimated that the total number of atoms in the visible universe is between 1078 and 1080. That lone bacterium can produce 1081 bacteria — more than the estimated number of atoms in the visible universe — in just 27 hours. Luckily, the conditions are nowhere close to ideal for bacteria.
Animal rights movements are explicitly about honoring the quality of life for animals. This means it is ethically, morally wrong – the animal rights activists say – to put these animals in zoos, use them for medical experiments, or stage fights where they kill one another for our entertainment (think Michael Vick). Ethologists have produced copious observations, some available in clips on YouTube, showing that animals are capable of suffering, compassionate behavior, grieving — and some animals, from magpies and foxes to elephants, have been observed "burying" their dead, under dirt or tall grasses.
Within our own species, we like to think we've reached the pinnacle: homo sapiens as the life form capable of the highest and most nuanced quality of life on the planet. And most of us believe that conditions which make it impossible or highly unlikely that a human can ever achieve the quality of life of which they're capable aren't just "social problems": they are immoral; they are evil. That's why we opposed slavery, finally gave women the right to vote, still struggle to be racially color-blind, and are beginning to understand that two people who love each other and want to make their commitment public should be encouraged -- not just permitted -- to marry, regardless of their sexual orientation.
We try to eliminate the obstacles to more complex species' attaining the highest quality of life because these obstacles are, for these species, anti-life. It's why we have stressed free public education, Medicare and Medicaid, Social Security, and why some other – perhaps more caring – developed nations provide health care, even free college. We are at the opposite end of the spectrum from bacteria.
For humans, one of the cruelest obstacles to ever attaining the quality of life that can fulfill our potential is too much quantity of life. Just see photos of the Favelas in Brazil, the slums in any major city, the people standing at intersections, begging for their day's food. Or see the tragedy of a fifteen-year-old girl with two children, pregnant again. One of the primary enemies of the quality of human life is too many children for mothers, too many people to stack up anywhere but in slums, too many illiterate and unskilled people because they must use all their energy merely to survive. For bacteria, as far as we can tell (or care), quantity of life is what it’s about. For human beings, quantity can easily kill quality.
This is why a safe abortion is often the only moral choice. Sex education could help tremendously, as could an economy that would take care of "the least among us," in the phrase attributed to Jesus. When cities like Rio de Janeiro, New York, Chicago, Calcutta and a thousand more show that overpopulation has metastasized into slums, quantity of life is killing quality of life. Scenes of a Pope addressing millions of poor and desperate people in Mexico City, telling them they must not use birth control – this is profound ignorance, but also profound evil. When children are having children and can't provide either for themselves or for their offspring, quantity of life is killing quality of life. And that is not just ugly; it's evil.
Nor could adoption solve this moral problem: first, because there aren’t enough people willing to adopt the number of uncared-for children in the world. But secondly, because it would turn poor women into de facto breeding stock for wealthier people. If we can’t care for the people already suffering, we have no moral right to set up systems guaranteeing to bring more into the world. The anti-abortion laws in some religions and countries are such systems. They harm people by not understanding that in all species we think of as "higher" — and certainly in our own species — worshiping the quantity of life kills the chance for the quality of life humans deserve.
This argument -- that conditions preventing quality of life are evil and must be opposed by all moral people and institutions -- was first and best framed by Pope Leo XIII in one of the most justly famous Catholic papal encyclicals in history: the Rerum Novarum of 1891. There, he was dealing with the widespread use of child labor, which forced people to live like "brutes" without the resources to rise above that low quality of life. That brilliant encyclical helped stop the use of child labor in many countries. In 1891, Pope Leo XIII didn't imagine that overpopulation -- more children than a mother, a city or a nation can support -- would become one of the most brutal and deadly impediments to humans living the quality of life for which they were created (or, which they reached through evolution).
Conservatives are right in saying that abortion must be seen as a moral issue, not merely "a woman’s right to choose." But in that moral issue, they have chosen the wrong side.