The mystery of the unity of the triune God is also grasped in reality by the sacramental union between man and woman. In no way is this unity made more visible than by having children. The Christian tradition has long seen in the natural family (i.e., husband, wife, and children) as an icon of the Trinity. The church teaches that raising children, at least as much as having them, is why Christ has raised marriage "to the dignity of a sacrament" (Canon 1055.1). So, a couple who, through no fault of their, own are unable to bear children, need not be discouraged because a valid sacramental marriage is open to them, too.
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Perhaps the biggest challenge to couples seeking to live marriage as a sacrament is faithfully living what the church teaches regarding sexuality and its necessary link to procreation. Nowhere more than here, at a significant crossroads where faith meets life, have so many Christians taken the worldly road. The term "birth control" is important because not only is the church not opposed to birth control, it teaches that limiting the number of children is a moral responsibility of all married couples. In his encyclical, Humanae Vitae, Pope Paul VI wrote that married couples need to arrive at a "full awareness of their obligations in the matter of responsible parenthood, which today, rightly enough, is much insisted upon, but which at the same time should be rightly understood" (par. 10). So, taking into account the "physical, economic, psychological and social conditions, responsible parenthood is exercised by those who prudently and generously decide to have more children, and by those who, for serious reasons and with due respect to moral precepts, decide not to have additional children for either a certain or an indefinite period of time" (par. 10). "From this it follows," the encyclical continues, "that they are not free to act as they choose in the service of transmitting life, as if it were wholly up to them to decide what is the right course to follow. On the contrary, they are bound to ensure that what they do corresponds to the will of God the Creator" (par. 10). The nature of Christian marriage as set forth in scripture "makes [God’s] will clear" and "the constant teaching of the Church spells it out" (par. 10). What the constant teaching of the church spells out is that the regulation of births, either by way of spacing or number, cannot be done morally by using artificial methods of contraception.