You are correct to point out the disconnect in what is allowed and what is not. My feeling is that children are a blessing from God - HE is who opens and closes the womb (Isa. 66:9). But, He expects us to act responsibly. There are many types of birth control that cause abortion (hormones and IUD) and some that don't (barrier methods, NPF, sterilization) though as you correctly said, they all have a chance of failure. My dad had a vasectomy after fathering five children but afterward he fathered two more!
I think ALL abortifacients SHOULD be rejected because they can cause a fertilized egg to not implant to the uterine wall and be expelled - the woman has no idea when it occurs - which cheapens the value of human life and blurs the line between it and abortion.
I agree on the abortifacients although I may define them a little differently than you do. I actually was not commenting on whether birth control is right or wrong, Just pointing out the disconnect in those who think preventing pregnancy is wrong unless you use their approved method to prevent it.