Those who bring forth children outside of marriage are parents too, it is true; but there is no honor in it. Therefore it is said, “Let the marriage bed be undefiled.” In other words, let it not be a whore’s bed or an adulterous bed…. St. Paul is … speaking of the purity which should obtain in the marriage estate, that married people should not be fornicators or adulterers and adulteresses. Whatever else occurs in the married state God covers up, for, after all, it is done in order to bring forth children and God approves of this, for it is his ordinance.
This kind of impurity {of fornication}, says God, I do not see {in marriage}. Here parents, fathers and mothers, or married people, are excused; God will not consider it impurity on account of inherited sin, nor will he consider it to be a sin. God will rather build his kingdom of heaven over this work and cover up everything that is unclean in it for the sake of his ordinance and creation. This is the kind of purity St. Paul means when he says: “Let the marriage bed be undefiled.” He is not talking about cleanness in eating and drinking, but rather about marital faithfulness and duty, in which one trusts the other, keeps away from all other persons, and is content with his own spouse; this is what he calls purity. (“Sermon at Marriage of Sigismund von Luindenau, 1545” Luther’s Works, v. 51, p.364-365)