Large Catechism: Necessity of Marriage (LC I 211-221)

In the second place, you must know also that marriage is not only an honorable but also a necessary state. In general and in all conditions it is solemnly commanded by God that men and women, who were created for marriage, shall be found in this estate. Yet there are some exceptions (although few) whom God has especially set apart. They are not fit for the married estate. Or there are individuals whom He has released by a high, supernatural gift so that they can maintain chastity without this estate. For where nature has its course—since it is given by God—it is not possible to remain chaste without marriage. For flesh and blood remain flesh and blood. The natural desire and excitement have their course without delay or hindrance, as everybody sees and feels. In order, therefore, that it may be easier in some degree to avoid inchastity, God has commanded the estate of marriage. In this way everyone may have his proper portion and be satisfied with it. Yet God’s grace is also required in order that the heart may be pure. (211-212)