From Casti Connubii §5:
… let it be repeated as an immutable and inviolable fundamental doctrine that matrimony was not instituted or restored by man but by God; not by man were the laws made to strengthen and confirm and elevate it but by God, the Author of nature, and by Christ Our Lord by Whom nature was redeemed, and hence these laws cannot be subject to any human decrees or to any contrary pact even of the spouses themselves. This is the doctrine of Holy Scripture; this is the constant tradition of the Universal Church; this the solemn definition of the sacred Council of Trent, which declares and establishes from the words of Holy Writ itself that God is the Author of the perpetual stability of the marriage bond, its unity and its firmness.
In §10 he quotes St. Augustine on the nature of that marriage bond (emphasis mine):
“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.”
§20 speaks directly to the severity of God’s laws concerning marriage (emphases mine):
Wherefore, conjugal faith, or honor, demands in the first place the complete unity of matrimony which the Creator Himself laid down in the beginning when He wished it to be not otherwise than between one man and one woman. And although afterwards this primeval law was relaxed to some extent by God, the Supreme Legislator, there is no doubt that the law of the Gospel fully restored that original and perfect unity, and abrogated all dispensations as the words of Christ and the constant teaching and action of the Church show plainly.
Cardinal Kasper ought to just have read Casti Cannubii to the recent consistory.