At the core of the anti-choice movement are the die-hard Catholics and others that believe that the Wages of Sin are bastard children. Unwanted pregnancy is God's punishment for irresponsible women fooling around. These are traditionalists, as hidebound and reactionary as the Taliban or the Wahabbi Muslims. Then there are a number of people who are less rigid, and some who are for allowing some, but not all abortions.
As for the possible calculation of evil, ie the evil of abortion vs. the evils of birth control, there would need to be a scientific Scale of Evil for that. I deem that this is impossible, since actual evil cannot be calculated, only the results of evil actions, which vary greatly.