Further, this reminder needs to be added, that there is some difference between the chastity of that conjugal intercourse which would have taken place before the Fall and that which now takes place in this corrupt nature after the Fall. Epiphanius says, Bk. 1, Tom. 3, Heresy 45, that appetites were implanted by God in nature, not for the purpose of unnatural conduct but for a good use and for a necessary and useful order. And he adds: “Therefore I have also said that the appetite of bodily desire is not something unnatural, for it has been given in order that children may be begotten in purity, in order that the command may be fulfilled: ‘Be fruitful and multiply.’ ” Therefore the desire of one sex for the other was found in nature also when it was whole, but there it would have been without the flame and fire of lust, completely holy and pure.The latter portion of the statement I placed in bold is obviously a reference to Hebrews 13:4 (which we have been discussing below). And here is an excerpt of the Luther reference given by Chemnitz at the end of the second paragraph above:
Now through sin this most noble work of generating children, which is truly a creation of God, has been infected with the leprosy of lust (as Luther calls it). And when Scripture calls conjugal cohabitation chastity and holiness, this is not to be understood as if the flames of lust were sanctified in spouses so that they are a good, pure, and holy thing. For our first parents immediately after the Fall covered the organs of generation because they sensed there a disorder to which they were before unaccustomed. And God Himself provided them garments. Paul also (1 Cor. 12:23) calls them our less honorable members, which are to be honored by covering them. One must therefore distinguish between the work itself, which is per se a good creation of God, and the vice and disorderliness of lust, which came to it through sin. For Scripture ascribes to spouses both lack of self-control (1 Cor. 7:5) and chastity (1 Tim. 2:15). However, that accompanying disorder is shielded and covered by the honor, sanctity, and purity of marriage on account of the institution and blessing of God, so that it is not imputed to believers; but conjugal cohabitation, even though it is not so pure as it would have been before the Fall (for it is not without the fire of lust) is nevertheless before God and in His sight chastity, holiness, and purity. For marriage is now, since the Fall, as it were an umbrella, by which many marital follies, if I may use that expression, are covered, so that they are not imputed to believers, as Luther beautifully argues on Gen. 26, fol. 382 [Luther’s Works, American Edition, 5, pp. 31–38].
In order therefore that pious spouses may preserve chastity in conjugal cohabitation, let them learn first of all to think [rightly] about the flames, tire, and ardor of lust; let them not think that these are per se a good thing and pleasing to God, but be sure that if they are confined within the limits of matrimony God does not impute them to believers but tolerates them, overlooks and ignores them, and as Luther beautifully accommodates the word used in Acts 13:18, “bears with them.” Second, spouses should also ponder what Paul says in 1 Thess. 4:4–5 [KJV]: “… that every one of you should know how to possess his vessel … not in the lust of concupiscence, even as the Gentiles.” For as Peter calls the wife the weak vessel, so this passage in Paul is not ineptly explained of conjugal intercourse, that pious spouses should not indulge with the furor of beastly lust, like heathen, but should use moderation and bridle the passions of lust in order that marriage may be honorable and the couch unpolluted. And where they have overdone it, they will pray that this may not be imputed but forgiven.
[Genesis 26:]8 When he had been there a long time, Abimelech king of the Philistines looked out of a window and saw Isaac fondling Rebecca his wife.
To be sure, marriage is not without its own imperfection and uncleanness; but God tolerates and overlooks this. If the natural order of procreating offspring is preserved, then God overlooks and forgives, or, as Luke expresses it in Acts (13:18) with a fine word, τροποφορει̂, He bears with them. He says: “I shall bear with this way for the sake of the preservation of the human race. I know that you were conceived in sins; but I forgive you, and I permit you to have your wife as a help. In her alone you should take delight, and you should bear it patiently, even if something sad or irksome happens. I, too, shall have patience, whether you embrace her during the night, when she is naked, or during the day, when she is clothed.”
Luther's Works, Vol. 5, Page 34