Is Humanae Vitae’s Teaching on Contraception Infallible?

To be precise, Bl. Pope Paul VI definitively reaffirms the Church’s long standing infallible teaching on contraception in his encyclical Humanae Vitae (HV). In other words, there was no need to make an ex cathedra definition in 1968 (see CCC 891).

This is clearly seen in Pope Paul VI’s matter-of-fact teaching in his encyclical:

This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act (HV 12; emphasis added).

For man cannot attain that true happiness for which he yearns with all the strength of his spirit, unless he keeps the laws which the Most High God has engraved in his very nature. These laws must be wisely and lovingly observed. On this great work, on all of you and especially on married couples, we implore from the God of all holiness and pity an abundance of heavenly grace as a pledge of which we gladly bestow our apostolic blessing (HV 31; emphasis added).

For more on this topic, see our related tract and our answer regarding medical treatment and the principle of double effect. A key issue re: double effect would be avoiding a treatment that had an abortifacient side effect.