Why I am pro-choice: Being pro-choice allows every woman to make the highly personal decision to terminate or keep a pregnancy. Given the wide variety of situations that individual women are in, I cannot presume to know what is best. Choice should be their right as a person.
Men have the choice to walk away from women they have had sex with. There are no biological consequences to this act. A woman cannot 'walk away' from a pregnancy. She can either choose to try to bring the baby to term or choose not to (note that not every pregnancy produces a live child-- miscarriage, accident, and severe birth defects are all things that are outside of the mother-to-be's control and may result in the loss of the baby).
In short, there is enough about having children that we as humans cannot control, but the choice to continue a pregnancy once it has begun should ultimately be made by the woman who will pay the biological price. There are as many reasons to have children or to not have children as there are women, men, and families. There is no one rule that will cover the the rich diversity of that experience. That is why I believe that choice is of paramount importance.