3 This we now say, that, according to this condition of being born and dying, which we know, and in which we have been created, the marriage of male and female is some good; the compact whereof divine Scripture so commends, as that neither is it allowed one put away by her husband to marry, so long as her husband lives: nor is it allowed one put away by his wife to marry another, unless she who have separated from him be dead. Therefore, concerning the good of marriage, which the Lord also confirmed in the Gospel, not only in that He forbade to put away a wife, save because of fornication, but also in that He came by invitation to a marriage, there is good ground to inquire for what reason it be a good.
And this seems not to me to be merely on account of the begetting of children, but also on account of the natural society itself in a difference of sex. Otherwise it would not any longer be called marriage in the case of old persons, especially if either they had lost sons, or had given birth to none. But now in good, although aged, marriage, albeit there has withered away the glow of full age between male and female, yet there lives in full vigor the order of charity between husband and wife: because, the better they are, the earlier they have begun by mutual consent to contain from sexual intercourse with each other: not that it should be matter of necessity afterwards not to have power to do what they would, but that it should be matter of praise to have been unwilling at the first, to do what they had power to do.
If therefore there be kept good faith of honor, and of services mutually due from either sex, although the members of either be languishing and almost corpse-like, yet of souls duly joined together, the chastity continues, the purer by how much it is the more proved, the safer, by how much it is the calmer. Marriages have this good also, that carnal or youthful incontinence, although it be faulty, is brought unto an honest use in the begetting of children, in order that out of the evil of lust the marriage union may bring to pass some good.
Next, in that the lust of the flesh is repressed, and rages in a way more modestly, being tempered by parental affection. For there is interposed a certain gravity of glowing pleasure, when in that wherein husband and wife cleave to one another, they have in mind that they be father and mother.
Source: On the Good of Marriage (New Advent)