9 Truly we must consider, that God gives us some goods, which are to be sought for their own sake, such as wisdom, health, friendship: but others, which are necessary for the sake of somewhat, such as learning, meat, drink, sleep, marriage, sexual intercourse. For of these certain are necessary for the sake of wisdom, as learning: certain for the sake of health, as meat and drink and sleep: certain for the sake of friendship, as marriage or sexual intercourse: for hence subsists the propagation of the human kind, wherein friendly fellowship is a great good.
These goods, therefore, which are necessary for the sake of something else, whoso uses not for this purpose, wherefore they were instituted, sins; in some cases venially, in other cases damnably. But whoso uses them for this purpose, wherefore they were given does well. Therefore, to whomsoever they are not necessary, if he use them not, he does better. Wherefore, these goods, when we have need, we do well to wish; but we do better not to wish than to wish: because ourselves are in a better state, when we account them not necessary.
And on this account it is good to marry, because it is good to beget children, to be a mother of a family: but it is better not to marry, because it is better not to stand in need of this work, in order to human fellowship itself. For such is the state of the human race now, that (others, who contain not, not only being taken up with marriage, but many also waxing wanton through unlawful concubinages, the Good Creator working what is good out of their evils) there fails not numerous progeny, and abundant succession, out of which to procure holy friendships.
Whence we gather, that, in the first times of the human race, chiefly for the propagation of the People of God, through whom the Prince and Saviour of all people should both be prophesied of, and be born, it was the duty of the Saints to use this good of marriage, not as to be sought for its own sake, but necessary for the sake of something else: but now, whereas, in order to enter upon holy and pure fellowship, there is on all sides from out all nations an overflowing fullness of spiritual kindred, even they who wish to contract marriage only for the sake of children, are to be admonished, that they use rather the larger good of continence.
Source: On the Good of Marriage (New Advent)