At Dean Nation, I've commented on the phenomenon of pharmacists refusing to fill birth-control prescriptions, citing their pro-life religious beliefs. Fundamentally, being anti-birth-control is completely at odds with the pro-life position, because birth control prevents undesired pregnancy, obviating the need for abortions (the exception is the morning-after type pills, which could legitimately be interpreted as an abortion pill if you define life as beginning at conception).
It's tempting to ascribe the religious Christian right's self-defeating take on birth control as stemming from their religious attitudes towards human sexuality. I have very little knowledge on this topic, but the stereotype of the Christian faith is a Puritan attitude, where sex is a sinful act. The repressive attitude towards the topic may even account for the profusion of cabarets and erotic clubs in the South - when I moved from Boston to Houston, I was struck by how much more blatant these types of establishments are in their advertising and presence.
As I confessed, though, I simply don't know enough about Christian theology to have an informed opinion on the real religious impetus behind why this hostility towards birth control arises from the pro-life crowd. I do know that in Islam, sex between married couples is considered a religious act, and earns the man and woman blessings. The specific issue of whether birth control is valid or not was one i was previously unaware had any controversy attached (though I laughed along with everyone else at the "every sperm is sacred" skit in Monty Python, I didn't think that was actually practiced).
What are the religious justifications for a ban on birth control? I can't answer, and I suspect that there might well be none. Ultimately, this issue is probably driven more by political extremism (complete with purifying litmus-test jihad) than by any rational adherence to religious precepts or a sincere desire to reduce abortions overall.
Certainly, if reduction of abortion was the real goal, then methods like birth control that actively promote the desired results would not be under attack. That they are, I think, suggests something.