The use of condoms to prevent transmission of a disease is intrinsically evil because the object freely chosen that specifies the moral nature of the act is not the marital act, an act in which husband wife give and receive one another and become literally “one flesh,” but a different kind of act, one that in no way unites them but rather changes utterly the “language of the body.”

by William E. May, Michael J. McGivney Professor of Moral Theology at the Pontifical John Paul II Institute for Studies on Marriage and Family