Having shown that the pretexts usually alleged for second marriages have no weight, St. Ambrose declares that he does not condemn them, though from the Apostle's words he sets forth their inconveniences, though the state of those twice married is approved in the Church, and he takes occasion to advert to those heretics who forbid them. And he says that it is because the strength of different persons varies that chastity is not commanded, but only recommended.
67. I say, then, that widows who have been in the habit of giving neither are in want of their necessary expenses, nor of help, who in very great dangers have often guarded the resources of their husbands; and further, I think that the good offices of a husband are usually made up for to them by sons-in-law and other relatives, and that God's mercy is more ready to help them, and therefore, when there is no special cause for marrying, the desire of so doing should not exist.
68. This, however, I say as a counsel, we do not order it as a precept, stirring up the wills of widows rather than binding them. For I do not forbid second marriages, only I do not advise them. The consideration of human weakness is one thing, the grace of chastity is another. I say more, I do not forbid second, but do not approve of often repeated marriages, for not everything is expedient which is lawful: “All things are lawful to me,” says the Apostle, “but all things are not expedient.” As, also, to drink wine is lawful, but, for the most part, it is not expedient.
69. It is then lawful to marry, but it is more seemly to abstain, for there are bonds in marriage. Do you ask what bonds? “The woman who is under a husband is bound by the law so long as her husband lives; but if her husband be dead she is loosed from the law of her husband.” It is then proved that marriage is a bond by which the woman is bound and from which she is loosed. Beautiful is the grace of mutual love, but the bondage is more constant. “The wife has not power of her own body, but the husband.” And lest this bondage should seem to be rather one of sex than of marriage, there follows: “Likewise, also, the husband has not power of his own body, but the wife.” How great; then, is the constraint in marriage, which subjects even the stronger to the other; for by mutual constraint each is bound to serve. Nor if one wishes to refrain can he withdraw his neck from the yoke, for he is subject to the incontinence of the other. It is said: “You are bought with a price, be not servants of men.” You see how plainly the servitude of marriage is defined. It is not I who say this, but the Apostle; or, rather, it is not he, but Christ, Who spoke in him. And he spoke of this servitude in the case of good married people. For above you read: “The unbelieving husband is sanctified by his believing wife; and the unbelieving wife by her believing husband.” And further on: “But if the unbelieving depart, let him depart. A brother or a sister is not bound in such cases.” If, then, a good marriage is servitude, what is a bad one, when they cannot sanctify, but destroy one another?
70. But as I exhort widows to keep the grace of their gift, so, too, I incite women to observe ecclesiastical discipline, for the Church is made up of all. Though it be the flock of Christ, yet some are fed on strong food, others are still nourished with milk, who must be on their guard against those wolves who are hidden in sheep's clothing, pretending to all appearance of continence, but inciting to the foulness of incontinence. For they know how severe are the burdens of chastity, since they cannot touch them with the tips of their fingers; they require of others that which is above measure, when they themselves cannot even observe any measure, but rather give way under the cruel weight. For the measure of the burden must always be according to the strength of him who has to bear it; otherwise, where the bearer is weak, he breaks down with the burden laid upon him; for too strong meat chokes the throats of infants.
71. And so as in a multitude of bearers their strength is not estimated by that of a few; nor do the stronger receive their tasks in accordance with the weakness of others, but each is allowed to bear as great a burden as he desires, the reward increasing with the increase of strength; so, too, a snare is not to be set for women, nor a burden of continence beyond their strength to be taken up, but it must be left to each to weigh the matter for herself, not compelled by the authority of any command, but incited by increase of grace. And so for different degrees of virtue a different reward is set forth, and one thing is not blamed that another may be praised; but all are spoken of, in order that what is best may be preferred.
Source: Concerning Widows (New Advent)