Groups of largely male pro life religious leaders (both Catholic and certain other religions) have determined that abortion is murder. The determination that it is murder is based on personal relationships the leaders claim to have with God, not on scientific fact. Differing opinions challenging those of the leaders, that lead to action, are not allowed. It is impossible for followers of these religious pro life leaders to concede to the pro choice side without being complicit in murder under church law or becoming excommunicated from the church. For true followers of these religious leaders, changing one’s opinion such that it is different from that of the church is impossible.

Latae Sententiae Excommunication
http://www.vatican.va/archive/ENG1104/__P57.HTM
http://en.wikipedia.org/wiki/Latae_sententiae
http://www.ewtn.com/expert/answers/abortio2.htm