My wife is latino and we occassionally go to spanish mass, and I think our (Mexican) priest summed it up rather well. 

During a sermon he was giving on abortion, he described it as nothing less than a diabolical abomination ('diabolical' and 'abomination' are the same words in spanish or english, and he used both these words).  He then went on to lament how over the years we have allowed so many of our future "doctors and musicians, artists and mathematicians" to be killed, and he wondered aloud what long term effect this would have on the future of our society. 

He continued, using as example the famous Christmas movie 'A Wonderful Life', where Jimmy Stewart's wish that he had not been born was temporarily granted, and Stewart discovered that his own brother (whose life Stewart had in fact saved by rescuing him after he fell through a frozen lake) had died as a child because Stewart was not around to pull him out when he fell through the ice.  Moreover since his brother had died, he in turn was not able to be the war hero he in fact was, because he was not around to save the hundreds of men on the ship that he otherwise would have captained, and consequently they had all died as well.  The point was that as a practical matter, no man is an island; every life is important, and if a particular person was not around, they would be missed somehow.

I know I have not done his presentation justice, but I thought Father Jairo really had an excellent, thought provoking way of putting it.

I don't think the utilitarian calculus brought up by hopeforall would convince the atheist or the agnostic.

Even if they do acknowledge the obvious truth that we're destroying millions of unique, unrepeatable and precious human people. 

Even if they do agree that some of them might have cured cancer, discovered new worlds, exercised the political acumen to unite humanity.

Many who do not believe in Jesus Christ will not embrace the reality of the pro-life cause because sooner or later, it will challenge them to explore the wrongness of their personal behavior.

Listen, I'm first in line to have my personal behavior recognized as  wrong.  But to one who does not acknowledge the merciful God who waits for His children to repent and turn to Him, it's too hard to deal with.

So I am less hopeful than hopeforall.