It's unfortunate that most people think of this as a black and white issue.
I would argue that most pro-choice activists dislike the concept of therapeutic abortions. This procedure is a relatively safe procedure for the mother, but it does come with complications. For the child/fetus/human/tissue/baby it also means the end of its life and thus the ethical implications of this procedure are very significant to most caring individuals, including both pro-choicers and pro-lifers. It would be much simpler if the pregnancies were just prevented.
However, once the pregnancy has happened, one has to somehow deal with it. Whether or not society objectifies young women or whether idiotic rags like Vogue make women too thin, etc. is largely irrelevant when one's sister or daughter or friend has already become pregnant. Here, the well-being of the young mother carrying this fetus is of prime importance, to the pro-choice crowd. One must take into account several factors, which includes the personal circumstances of the mother, including the psychological and physical condition of that person, as well as the financial situation. While the wishes of the father are perhaps something to be considered, it's sobering to note that often they are a non-issue, because the father couldn't care less or elsse is encouraging the abortion.
I'm sure every pro-choicer would be overjoyed if abortion never had to be performed. However, it's extremely difficult to educate these women to prevent pregnancy, when such information is considered taboo in our schools, etc, and when money for such preventative measures doesn't exist. I always find it ironic that the forces that wish to completely stop abortion, also are often less willing to provide funding for education and birth control to decrease the need for abortion in the first place.
This is a very difficult issue, and I hope people give it the true consideration it deserves. Try to put yourselves in the shoes of these women, and just try to imagine what they are going through when they make this very difficult decision. When the time comes that it's your daughter perhaps then you can understand that while you may think it's wrong, they may still need your support one way or another.