5 Wherefore also when he was advising married persons not to defraud one another of the due of carnal intercourse; lest, by this means, the one of them, (the due of marriage being denied to him,) being through his own incontinence tempted of Satan, should fall away into fornication, he says, “But this I say of leave, not of command; but I would that all men were as I myself; but each one has his own proper gift from God; but one in this way, and another in that.” You see that wedded chastity also, and the marriage faith of the Christian bed, is a “gift,” and this of God; so that, when as carnal lust exceeds somewhat the measure of sensual intercourse, beyond what is necessary for the begetting of children, this evil is not of marriage, but venial by reason of the good of marriage.
For not concerning marriage, which is contracted for the begetting of children, and the faith of wedded chastity, and the sacrament (indissoluble, so long as both live) of matrimony, all which are good; but concerning that immoderate use of the flesh, which is recognized in the weakness of married persons, and is pardoned by the intervention of the good of marriage, the Apostle says, “I speak of leave, not of command.” Also, when he says, “The woman is bound, so long as her husband lives: but, in case her husband shall have died, she is set free: let her be married to whom she will, only in the Lord: but she shall be more blessed, if she shall have so continued, according to my counsel;” he shows sufficiently that a faithful woman is blessed in the Lord, even when she marries a second time after the death of her husband, but that a widow is more blessed in the same Lord; that is, to speak not only in the words, but by instances also, of the Scriptures, that Ruth is blessed, but that Anna is more blessed.
Source: On the Good of Widowhood (New Advent)