10 Now when We come to explain, Venerable Brethren, what are the blessings that God has attached to true matrimony, and how great they are, there occur to Us the words of that illustrious Doctor of the Church whom We commemorated recently in Our Encyclical Ad salutem on the occasion of the fifteenth centenary of his death: "These," says St. Augustine, "are all the blessings of matrimony on account of which matrimony itself is a blessing; offspring, conjugal faith and the sacrament." And how under these three heads is contained a splendid summary of the whole doctrine of Christian marriage, the holy Doctor himself expressly declares when he said: "By conjugal faith it is provided that there should be no carnal intercourse outside the marriage bond with another man or woman; with regard to offspring, that children should be begotten of love, tenderly cared for and educated in a religious atmosphere; finally, in its sacramental aspect that the marriage bond should not be broken and that a husband or wife, if separated, should not be joined to another even for the sake of offspring. This we regard as the law of marriage by which the fruitfulness of nature is adorned and the evil of incontinence is restrained."
Source: Casti Connubii (Vatican.va)