“For the sage says that parents provide goods and houses for their children, but a wife is given by God alone [Prov. 19:14], everyone according to his need, just as Eve was given to Adam by God alone. And true though it is that because of excessive lust of the flesh lighthearted youth pays scant attention to these matters, marriage is nevertheless a weighty matter in the sight of God. For it was not by accident that Almighty God instituted the estate of matrimony only for man and above all animals, and gave such forethought and consideration to marriage. To the other animals God says quite simply, “Be fruitful and multiply” [Gen. 1:22]. It is not written that he brings the female to the male. Therefore, there is no such thing as marriage among animals. But in the case of Adam, God creates for him a unique, special kind of wife out of his own flesh. He brings her to him, he gives her to him, and Adam agrees to accept her. Therefore, that is what marriage is.
A woman is created to be a companionable helpmeet to the man in everything, particularly to bear children. And that still holds good, except that since the fall marriage has been adulterated with wicked lust. And now [i.e., after the fall] the desire of the man for the woman, and vice versa, is sought after not only for companionship and children, for which purposes alone marriage was instituted, but also for the pursuance of wicked lust, which is almost as strong a motive” (Luther’s Works, vol. 44, p.8).