Nobody wants to have an abortion. But we have to uphold a woman's right to make this decision for herself. It's a matter of free choice. To this may be added a few points about whether a fetus is a human being, the problem over-population and so forth. But the bottom line is the defense of choice and freedom.
Abortion is upheld on the principle of freedom of choice.
To deny women the freedom to abort is somehow to seen as putting the axe to freedom itself. Abortion involves violence and death. No one denies this. That's the whole point of abortion. We all know this, even the most militant pro-choice activist.