Adultery includes all unchastity. …
And especially needed nowadays-
Married life is not a subject for jest, but an excellent estate. God bestowed upon it all the goods of the earth, as is written in Gen. 1 [:28]. He honored it so highly that he reposed everything in it; for what he is concerned with here is that people should be raised up. Therefore, let each one see to it that he remains with his wife and vice versa, and that both keep their bodies pure, not only outwardly but also that you may not set your heart upon another. They shall “become one flesh,” it is said [Gen. 2:24; Matt. 19:6], and this they do on account of the first commandment: Fear God! What you must say is: Even though I do have chances to kick over the traces, nevertheless, since God says, Fear me! I will not do it. Even though the emperor will not find it out, God who is above me will. Therefore say, O my God, grant me grace that I may not fall and that I may keep my marriage pure.
Luther sums up-
This means that you are to live chastely in your marriage, in body, words, gestures, and heart. That’s why God gave to each his wife.
People need to pay attention. Luther’s right.