Ever read Augustine's De Genisi Ad Litteram? There's a bit in it where he says that if a Christian tells a non-Christian something that the non-Christian knows is wrong, then the non-Christian will reject everything else the Christian has to say. 

When Humanae Vitae says that "natural law" says that procreation is the primary reason for having sexual relations, people will say "the unmarried man who wrote that doesn't know what he is talkong about. When the magisterium tells us that God is sexist, rational people will reject either God or the magisterium. When traditionalists say that the magisterium is to make moral decisions for us, it is clear that the traditionalists have a faith tjat is childish  (not child-like, childish).