Personally, I am pro choice because I see the alternative as an even greater moral crime against the unborn child.
Abortion - especially when safe, legal, and destigmatised - lets women who become pregnant as result of rape, assault and forced marriage to return to their chosen path in life, allowing them freedom and a chance at happiness.
In addition, I am a great believer in the value of human life, so I do not think that the creation of a new person should be taken lightly. Therefore, I see the abortion of an "unwanted" child as mercy on the parent's part. Anyone who has seen the lifelong psychological damage that broken or unhealthy homes does to people, will understand that terminating the pregnancy can be the more favourable alternative, as opposed to a life of abuse and sorrow.
When safe, legal abortion, is not an option, many turn to unsafe, illegal alternatives. If they fail, the child and mother are both condemned to a life of misery, and if they both die in the process, two lives have been thrown away. If they succeed, the woman may end up with a distrust of the system that failed to provide her with this service, thus developing less respect for the law in other areas, and becoming more likely to seek extralegal alternatives to her problems.
However, I understand that some may be worried that widely available abortion will encourage sleeping around and diminish use of contraceptives. The best way to combat this is full and comprehensive sexual education that will not only impress upon adolescents the necessity of contraceptives and the gravity of unwanted pregnancy and ensuing abortions, but also provide services for these young people.
This is just my hastily written two pennies' worth :)