Why Contraception is Wrong - Pope John Paul II. Familiaris Consortio: An Apostolic Exhortation on the integral role of the Christian family in the modern world.

It is precisely by moving [away] from “an integral vision of man and of his vocation, not only his natural and earthly, but also his supernatural and eternal vocation,” that Paul VI affirmed that the teaching of the Church “is founded upon the inseparable connection, willed by God and unable to be broken by man on his own initiative, between the two meanings of the conjugal act: the unitive meaning and the procreative meaning.” And he concluded by re-emphasizing that there must be excluded as intrinsically immoral “every action which, either in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible.”
When couples, by means of recourse to contraception, separate these two meanings that God the Creator has inscribed in the being of man and woman and in the dynamism of their sexual communion, they act as “arbiters” of the divine plan and they “manipulate” and degrade human sexuality—and with it themselves and their married partner—by altering its value of “total” self-giving. Thus, the innate language that expresses
the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life, but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality.