What if Luther had foreseen today's anti-child contraceptive culture and thought to include Scriptural teaching on it in his catechisms? What would it look like in that context? Well, here's what I think he would have written - in fact he did write most of what is below. The blue text would be for the Small Catechism, and the rest of the text would be his longer explanation for the Large Catechism.
The First Divine Ordinance
Be fruitful and multiply. [Genesis 1:28, 9:1, 9:7, 35:11, Jeremiah 23:3]
What does this mean?
We should fear, love, and trust in God above all things so that we do not despise His gift of children, but rather welcome them with praise and thanksgiving as an heritage unto the Lord and as a reward and blessing, and raise and nurture them in the admonition of the Lord.
“For this word which God speaks, 'Be fruitful and multiply,' is not a command. It is more than a command, namely, a divine ordinance which it is not our prerogative to hinder or ignore. Rather, it is just as necessary as the fact that I am a man, and more necessary than sleeping and waking, eating and drinking, and emptying the bowels and bladder. It is a nature and disposition just as innate as the organs involved in it. Therefore, just as God does not command anyone to be a man or a woman but created them the way they have to be, so he does not command them to multiply but creates them so that they have to multiply. And wherever men try to resist this, it remains irresistible nonetheless and goes its way through fornication, adultery, and secret sins, for this is a matter of nature and not of choice.
. . . from this ordinance of creation God has himself exempted three categories of men, saying in Matthew 19:12, 'There are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven.' Apart from these three groups, let no man presume to be without a spouse. And whoever does not fall within one of these three categories should not consider anything except the estate of marriage. Otherwise it simply impossible for you to remain righteous. For the Word of God which created you and said, 'Be fruitful and multiply,' abides and rules within you; you can by no means ignore it, or you will be bound to commit heinous sins without end. [Luther's Works, vol. 45, page 15 ff]
1 Timothy 2:15: “But women will be saved through bearing children—if they continue in faith, love and holiness with propriety.”
“That subjection of women and domination of men have not been taken away, have they? No. The penalty remains. The blame passed over. The pain and tribulation of childbearing continue. Those penalties will continue until judgment. So also the dominion of men and the subjection of women continue. You must endure them. You will also be saved if you have also subjected yourselves and bear your children with pain. 'THROUGH BEARING CHILDREN.' It is a very great comfort that a woman can be saved by bearing children, etc. That is, she has an honorable and salutary status in life if she keeps busy having children. We ought to recommend this passage to them, etc. She is described as 'saved' not for freedom, for license, but for bearing and rearing children. Is she not saved by faith? He goes on and explains himself: bearing children is a wholesome responsibility, but for believers. To bear children is acceptable to God. He does not merely say that bearing children saves: he adds: if the bearing takes place in faith and love, it is a Christian work, for to the pure all things are pure (Titus 1 :15). Also: 'All things work together,' Rom. 8:28. This is the comfort for married people in trouble: hardship and all things are salutary, for through them they are moved forward toward salvation and against adultery.... 'IN FAITH.' Paul had to add this, lest women think that they are good in the fact that they bear children. Simple childbearing does nothing, since the heathen also do this. But for Christian women their whole responsibility is salutary. So much the more salutary, then is bearing children. I add this, therefore, that they may not feel secure when they have no faith." [Luther's Works, Vol. 28, p. 279]
“For one must also consider that at that time fertility was regarded as an extraordinary blessing and a special gift of God, as is clear from Deut. 28:4, where Moses numbers fertility among the blessings. ‘There will not be a barren woman among you,’ he says (cf. Ex. 23:26). We do not regard this so highly today. Although we like and desire it in cattle, yet in the human race there are few who regard a woman’s fertility as a blessing. Indeed, there are many who have an aversion for it and regard sterility as a special blessing. Surely this is also contrary to nature. Much less is it pious and saintly. For this affection has been implanted by God in man’s nature, so that it desires its increase and multiplication. Accordingly, it is inhuman and godless to have a loathing for offspring. Thus someone recently called his wife a sow, since she gave birth rather often. The good-for-nothing and impure fellow! The saintly fathers did not feel like this at all; for they acknowledged a fruitful wife as a special blessing of God and, on the other hand, regarded sterility as a curse. And this judgment flowed from the Word of God in Gen. 1:28, where He said: ‘Be fruitful and multiply.’ From this they understood that children are a gift of God.” [Luther's Works, vol. 5, p. 329]
Genesis 38:9-10 “But Onan knew that the heir would not be his; and it came to pass, when he went in to his brother’s wife, that he emitted on the ground, lest he should give an heir to his brother. And the thing which he did displeased the LORD; therefore He killed him also.”
“Onan must have been a malicious and incorrigible scoundrel. This is a most disgraceful sin. It is far more atrocious than incest and adultery. We call it unchastity, yes, a Sodomitic sin. For Onan goes in to her; that is, he lies with her and copulates, and when it comes to the point of insemination, spills the semen, lest the woman conceive. Surely at such a time the order of nature established by God in procreation should be followed. Accordingly, it was a most disgraceful crime to produce semen and excite the woman, and to frustrate her at that very moment. He was inflamed with the basest spite and hatred. Therefore he did not allow himself to be compelled to bear that intolerable slavery. Consequently, he deserved to be killed by God. He committed an evil deed. Therefore God punished him.” [Luther's Works, vol. 7, page 20]
Genesis 30:2 "Jacob’s anger was kindled against Rachel, and he said: 'Am I in the place of God, who has withheld from you the fruit of the womb?'"
“Although it is very easy to marry a wife, it is very difficult to support her along with the children and the household. Accordingly, no one notices this faith of Jacob. Indeed, many hate fertility in a wife for the sole reason that the offspring must be supported and brought up. For this is what they commonly say: ‘Why should I marry a wife when I am a pauper and a beggar? I would rather bear the burden of poverty alone and not load myself with misery and want.’ But this blame is unjustly fastened on marriage and fruitfulness. Indeed, you are indicting your unbelief by distrusting God’s goodness, and you are bringing greater misery upon yourself by disparaging God’s blessing. For if you had trust in God’s grace and promises, you would undoubtedly be supported. But because you do not hope in the Lord, you will never prosper.” [Luther's Works, vol. 5, page 332]
Psalm 127: “Except the LORD build the house, they labor in vain that build it: except the LORD keep the city, the watchman waketh but in vain. It is vain for you to rise up early, to sit up late, to eat the bread of sorrows: for so he giveth his beloved sleep. Lo, children are an heritage of the LORD: and the fruit of the womb is his reward. As arrows are in the hand of a mighty man; so are children of the youth. Happy is the man that hath his quiver full of them: they shall not be ashamed, but they shall speak with the enemies in the gate.”