18 Nor must We omit to remark, in fine, that since the duty entrusted to parents for the good of their children is of such high dignity and of such great importance, every use of the faculty given by God for the procreation of new life is the right and the privilege of the married state alone, by the law of God and of nature, and must be confined absolutely within the sacred limits of that state.
Source: Casti Connubii (Vatican.va)