I did a bit of reading on
catholicsforchoice.org because this interests me.
It seems like, as you said, the legal aspect is a huge part of it too and for me that tends to carry more weight. It’s the same reason why I voted to allow Same Sex marriages in California. Pro-Choice Catholics might 100% agree with the catechism but still feel that in this country, people get to chose things for themselves: “Prevention Not Prohibition.” And I’d have to agree to an extent. Some people seem obsessed with outlawing abortion when their son or daughter is getting themselves into the same trouble because they are distracted by their cause. To me it seems like the Sola Fide and Salvation by Works debate, both sides are basically taking about the same thing, but arrive at the conclusion via a different route.
So, if a Pro-Choice Catholic agrees 100% with the catechism, your theological debate moves on to a different topic.