Martin Luther hits the nail squarely on the head, in his commentary on the Sixth Commandment in the Large Catechism. Oh, by the way, that new edition of the Book of Concord so many of you like…we didn’t produce it for people to use it as a trophy on the shelf, or a club to beat others over the head with in a game of, “I’m more confessional than you.” We produced it so that we all more easily would be able to use it and personally take it to heart. So…take this to heart.
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 [as with a rampart] every spouse that no one trespass against them.
Hear the Word of God, 1 Corinthians 6:
Flee from sexual immorality. Every other sina person commits is outside the body, but the sexually immoral person sins against his own body. Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, for you were bought with a price. So glorify God in your body.