What Pope Benedict XVI said on the 40th Anniversary of Humanae Vitae
"...Indeed, having received the gift of love, husband and wife are called in turn to give themselves to each other without reserve. Only in this way are the acts proper and exclusive to spouses truly acts of love which, while they unite them in one flesh, build a genuine personal communion. Therefore, the logic of the totality of the gift intrinsically configures conjugal love and, thanks to the sacramental outpouring of the Holy Spirit, becomes the means to achieve authentic conjugal charity in their own life.
The possibility of procreating a new human life is included in a married couple's integral gift of themselves. Since, in fact, every form of love endeavours to spread the fullness on which it lives, conjugal love has its own special way of communicating itself: the generation of children. Thus it not only resembles but also shares in the love of God who wants to communicate himself by calling the human person to life. Excluding this dimension of communication through an action that aims to prevent procreation means denying the intimate truth of spousal love, with which the divine gift is communicated: "If the mission of generating life is not to be exposed to the arbitrary will of men, one must necessarily recognize insurmountable limits to the possibility of man's domination over his own body and its functions; limits which no man, whether a private individual or one invested with authority, may licitly surpass" (Humanae Vitae, n. 17). This is the essential nucleus of the teaching that my Venerable Predecessor Paul VI addressed to married couples and which the Servant of God John Paul ii, in turn, reasserted on many occasions, illuminating its anthropological and moral basis.
Forty years after the Encyclical's publication we can understand better how decisive this light was for understanding the great "yes" that conjugal love involves. In this light, children are no longer the objective of a human project but are recognized as an authentic gift, to be accepted with an attitude of responsible generosity toward God, the first source of human life. This great "yes" to the beauty of love certainly entails gratitude, both of the parents in receiving the gift of a child, and of the child himself, in knowing that his life originates in such a great and welcoming love.
It is true, moreover, that serious circumstances may develop in the couple's growth which make it prudent to space out births or even to suspend them. And it is here that knowledge of the natural rhythms of the woman's fertility becomes important for the couple's life. The methods of observation which enable the couple to determine the periods of fertility permit them to administer what the Creator has wisely inscribed in human nature without interfering with the integral significance of sexual giving. In this way spouses, respecting the full truth of their love, will be able to modulate its expression in conformity with these rhythms without taking anything from the totality of the gift of self that union in the flesh expresses. Obviously, this requires maturity in love which is not instantly acquired but involves dialogue and reciprocal listening, as well as a special mastery of the sexual impulse in a journey of growth in virtue."