Thou shalt not commit adultery
The Sixth through Tenth Commandments “are easily understood from [the explanation of] the preceding; for they are all to the effect that we [be careful to] avoid doing any kind of injury to our neighbor.”
On its face, the Sixth Commandment deals with the issue of adultery. However, in true Luther style, we see that this commandment deals with so much more – namely the sanctity of marriage and the obligations of husband to wife and vice-versa. In a time when marriage seems to be disposable and the lines between who can be “married” have been substantially blurred (if not erased all together), the Lutheran Confessions as contained in the Book of Concord (into which the Small and Large Catechisms are incorporated) to which all Lutheran churches use (along with Scripture) as the basis of our theology and teaching, speak of marriage between one man and one woman in a monogamous, life-long, committed relationship sanctioned by God as bringing glory and honor to the God who created us.
From Luther’s Small Catechism
Q. What does this mean?
A. We must fear and love God, so that our words and actions will be clean and decent and so that everyone will love and honor their spouses.
Excerpt From Luther’s Large Catechism*
201] And it really aims at adultery, because among the Jews it was ordained and commanded that everyone 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.
202] 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; 203] 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. 204] 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. 205] Thus, to state it in the briefest manner, there is required this much, that everyone both live chastely himself and help his neighbor do the same, so that God by this commandment wishes to hedge round about and protect [as with a rampart] every spouse that no one trespass against them.
206] 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. 207] 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 [legitimately] live together, be fruitful, beget children, and nourish and train them to the honor of God.
Large Catechism Text in the public domain. http://bookofconcord.org/lc-3-tencommandments.php