Some difficulties arise when considering the 1930 encyclical letter of Pope Pius XI, Casti connubii, and the 1997 Vademecum for Confessors in light of the consistent teaching of the magisterium on the intrinsic evil of every contraceptive act. One difficulty is how to reconcile certain teachings of these two documents, which clearly allow for some sort of cooperation with a spouse who voluntarily renders the marital act infecund, with the absolute prohibition against formally acting in a contraceptive manner. The author (...) provides a care­ful reading of the documents that takes into account related magisterial and curial decisions, and shows that the documents reveal a consistent teaching. The teaching permits a certain limited cooperation with a spouse who renders the marital act infecund in a so-called natural way (e.g., natural onanism, or withdrawal) but excludes active material cooperation in sexual acts with a spouse who employs a condom or its equivalent. This teaching has relevance to recent debates about the prophylactic use of condoms among spouses. National Catholic Bioethics Quarterly 11.2 (Summer 2011): 301–328. (shrink)