One's view on abortion is a proxy for a other values – not that people on either side don’t sincerely hold their views, but that a whole array of our sort of people/not our sort of people issues come into it as well. In simplest form, being prochoice is seen as being for women, in some way. Being prolife is seen as being for churchy stuff. Given the wide difference among women and among churches, it rapidly gets messy. People resent being seen as anti X, just because they are for Y, when X and Y are not true opposites.
But this does explain why animal-rights activists, who by logic should be prolife extremists, tend to be prochoice. It's an Our Sort/Not Our Sort thing. This is intensified by the tendency of PETA folk to have few or no children, lining them up with the environmentalists. Their attempt at legacy is diverted to the entire planet. They don’t have the historically usual understanding about nurturance, and so adopt the faux-nurturance of Gaiaism. (four vowels in a row!).