12 But since, as the Lord says, “Not all receive this word;” therefore let her who can receive it, receive it; and let her, who contains not, marry; let her, who has not begun, deliberate; let her, who has undertaken it, persevere; let there be no occasion given unto the adversary, let there be no oblation withdrawn from Christ. Forsooth in the marriage bond if chastity be preserved, condemnation is not feared; but in widowed and virginal continence, the excellence of a greater gift is sought for: and, when this has been sought, and chosen, and by debt of vow offered, from this time not only to enter upon marriage, but, although one be not married, to wish to marry is matter of condemnation.
For, in order to show this, the Apostle says not, “When they shall have lived in delights, in Christ” they marry; but “they wish to marry; having,” says he, “condemnation, in that they have made of none effect their first faith,” although not by marrying, yet by wishing; not that the marriages even of such are judged matter of condemnation; but there is condemned a wrong done to purpose, there is condemned a broken faith of vow, there is condemned not a relief by lower good, but a fall from higher good: lastly, such are condemned, not because they have entered upon marriage faith afterwards, but because they have made of none effect the first faith of continence.
And in order to suggest this in few words, the Apostle would not say, that they have condemnation, who after purpose of greater sanctity marry, (not because they are not condemned, but lest in them marriage itself should be thought to be condemned:) but, after he had said, “they wish to marry,” he straightway added, “having condemnation.” And he stated the reason, “in that they have made of none effect their former faith,” in order that it may appear that it is the will which fell away from its purpose, which is condemned, whether marriage follow, or fail to follow.
Source: On the Good of Widowhood (New Advent)