“In the third place, 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 is 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.
Don’t let yourself be fooled on this score, even if you should make ten oaths, vows, covenants, and adamantine or ironclad pledges. For as you cannot solemnly promise that you will not be a man or a woman (and if you should make such a promise it would be foolishness and of no avail since you cannot make yourself something other than what you are), so you cannot promise that you will not produce seed or multiply, unless you belong to one of the three categories mentioned above. And should you make such a promise, it too would be foolishness and of no avail, for to produce seed and to multiply is a matter of God’s ordinance [geschöpffe], not your power.
From this you can now see the extent of the validity of all cloister vows. No vow of any youth or maiden is valid before God, except that of a person in one of the three categories which God alone has himself excepted. Therefore, priests, monks, and nuns are duty-bound to forsake their vows whenever they find that God’s ordinance to produce seed and to multiply is powerful and strong within them. They have no power by any authority, law, command, or vow to hinder this which God has created within them. If they do hinder it, however, you may be sure that they will not remain pure but inevitably besmirch themselves with secret sins or fornication. For they are simply incapable of resisting the word and ordinance of God within them. Matters will take their course as God has ordained” (Luther’s Works, vol. 45, p.18-19).