"There is not one argument against abortion, and a different argument against contraception. Both contraception and abortion violate the same principle of the natural law moral law; namely, It is never lawful to exercise an act and reject the end for which the act is designed.
The conjugal act is designed for the generation of new human life. Anything that frustrates that end is against both the natural moral law and the Divine Law. Both contraception and abortion frustrate the end of the marriage act, thus both contraception and abortion are grievously immoral for the exact same reason."--John Vennari
The above excerpt is from this article:Sarah Palin and Contraception. I am happy to recommend it.