From Luther's exposition of the 6th Commandment, "You shall not commit adultery", in the Large Catechism, a confessional document of the Lutheran church:
"These commandments now are easily understood from the preceding; for they are all to the effect that we avoid doing any kind of injury to our neighbor. But they are arranged in fine order. In the first place, they treat of his own person. Then they proceed to the person nearest him, or the closest possession next after his body, namely, his wife, who is one flesh and blood with him, so that we cannot inflict a higher injury upon him in any good that is his. Therefore it is explicitly forbidden here to bring any disgrace upon him in respect to his wife. And it really aims at adultery, because among the Jews it was ordained and commanded that every one must be married. Therefore also the young were early provided for [married], so that the virgin state was held in small esteem, neither were public prostitution and lewdness tolerated (as now). Therefore adultery was the most common form of unchastity among them.
But because among us there is such a shameful mess and the very dregs of all vice and lewdness, this commandment is directed also against all manner of unchastity, whatever it may be called; and not only is the external act forbidden, but also every kind of cause, incitement, and means, so that the heart, the lips, and the whole body may be chaste and afford no opportunity, help, or persuasion to inchastity. And not only this, but that we also make resistance, afford protection and rescue wherever there is danger and need; and again, that we give help and counsel, so as to maintain our neighbor's honor. For whenever you omit this when you could make resistance, or connive at it as if it did not concern you, you are as truly guilty as the one perpetrating the deed.Thus, to state it in the briefest manner, there is required this much, that every one both live chastely himself and help his neighbor do the same, so that God by this commandment wishes to hedge round about and protect every spouse that no one trespass against them.
But since this commandment is aimed directly at the state of matrimony and gives occasion to speak of the same, you must well understand and mark, first, how gloriously God honors and extols this estate, inasmuch as by His commandment He both sanctions and guards it. He has sanctioned it above in the Fourth Commandment: Honor thy father and thy mother; but here He has (as we said) hedged it about and protected it. Therefore He also wishes us to honor it, and to maintain and conduct it as a divine and blessed estate; because, in the first place, He has instituted it before all others, and therefore created man and woman separately (as is evident), not for lewdness, but that they should live together, be fruitful, beget children, and nourish and train them to the honor of God.
Therefore God has also most richly blessed this estate above all others, and, in addition, has bestowed on it and wrapped up in it everything in the world, to the end that this estate might be well and richly provided for. Married life is therefore no jest or presumption; but it is an excellent thing and a matter of divine seriousness. For it is of the highest importance to Him that persons be raised who may serve the world and promote the knowledge of God, godly living, and all virtues, to fight against wickedness and the devil."
The translation is that of the old Triglotta text from 1921. More could be said,of course, but this is a good place to start.