Why is Natural Family Planning (NFP) accepted by the Church while contraception is condemned? They both do the same thing--prevent pregnancy.

Because you don't judge the morality of actions by their effects or consequences. You judge their morality by what they essentially are. Using contraceptives such as condoms or diaphrams may accomplish the same end result as NFP, but the ways they go about it are very different.

First, NFP isn't contraception. In contraception an action is taken which prevents conception. In NFP, no such action occurs. Instead, sexual relations are avoided when conception is likely to occur.

Second, contraception attacks the good of procreation. It acts directly against procreation because some action is taken to thwart that good. Contraception treats procreation as an evil--as something to be acted against. NFP treats procreation as a good, but not one which necessarily needs to be pursued right now.

Third, contraception violates the natural link between the procreative and unitive aspects of the marital act. This link, as Humanae Vitae teaches, is established by God and may not be broken by man on his own initiative (Humanae Vitae, no. 12). NFP doesn't alter the marital act in any way.