What are the “Infallible” statements which refute NFP?

Catholic dogma teaches us that the primary purpose of marriage (and the conjugal act) is the procreation and education of children.

Pope Pius XI, Casti Connubii (# 17), Dec. 31, 1930: “The primary end of marriage is the procreation and the education of children.” Pope Pius XI, Casti Connubii (# 54), Dec. 31, 1930: “Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural powers and purpose sin against nature and commit a deed which is shameful and intrinsically vicious.”

Besides this primary purpose, there are also secondary purposes for marriage, such as mutual aid, the quieting of concupiscence and the cultivating of mutual love. But these secondary purposes must always remain subordinate to the primary purpose of marriage (the procreation and education of children). This is the key point to remember in the discussion on NFP.

Pope Pius XI, Casti Connubii (# 59), Dec. 31, 1930: “For in matrimony as well as in the use of the matrimonial right there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider SO LONG AS THEY ARE SUBORDINATED TO THE PRIMARY END and so long as the intrinsic nature of the act is preserved.”

Therefore, even though NFP does not directly interfere with the marriage act itself, as its defenders love to stress, it makes no difference. NFP is condemned because it subordinates the primary end (or purpose) of marriage and the marriage act (the procreation and education of children) to the secondary ends. NFP subordinates the primary end of marriage to other things, by deliberately attempting to avoid children (i.e., to avoid the primary end) while having marital relations. NFP therefore inverts the order established by God Himself. It does the very thing that Pope Pius XI solemnly teaches may not lawfully be done. And this point crushes all of the arguments made by those who defend NFP; because all of the arguments made by those who defend NFP focus on the marriage act itself, while they blindly ignore the fact that it makes no difference if a couple does not interfere with the act itself if they subordinate and thwart the primary PURPOSE of marriage. So, to summarize: the consistent teaching of the Magisterium, as expressed by Pius XI in Casti Connubii, that the primary end of marriage is the procreation of children and that all other ends must be subordinated to this, refutes NFP.