“It {Marriage} is an outward and spiritual sign of the greatest, holiest, worthiest, and noblest thing that has ever existed or ever will exist: the union of the divine and human natures in Christ. The holy apostle Paul says that as man and wife united in the estate of matrimony are two in one flesh, so God and man are united in the one person Christ, and so Christ and Christendom are one body. It is indeed a wonderful sacrament*, as Paul says [Eph. 5:32], that the estate of marriage truly signifies such a great reality. Is it not a wonderful thing that God is man and that he gives himself to man and will be his, just as the husband gives himself to his wife and is hers? But if God is ours, then everything is ours.
Consider this matter with the respect it deserves. Because the union of man and woman signifies such a great mystery, the estate of marriage has to have this special significance. This means that the wicked lust of the flesh, which nobody is without, is a conjugal obligation and is not reprehensible when expressed within marriage, but in all other cases outside the bond of marriage, it is mortal sin. In a parallel way the holy manhood of God covers the shame of the wicked lust of the flesh. Therefore, a married man should have regard for such a sacrament*, honor it as sacred, and behave properly in marital obligations, so that those things which originate in the lust of the flesh do not occur [among us] as they do in the world of brute beasts” (Luther’s Works, vol. 44, p.10).
* Written in 1519, Luther uses the word “sacrament” in a wide sense to mean an estate instituted by God, not that marriage gives forgiveness.