18 For what food is unto the conservation of the man, this sexual intercourse is unto the conservation of the race: and both are not without carnal delight: which yet being modified, and by restraint of temperance reduced unto the use after nature, cannot be lust. But what unlawful food is in the supporting of life, this sexual intercourse of fornication or adultery is in the seeking of a family. And what unlawful food is in luxury of belly and throat, this unlawful intercourse is in lust that seeks not a family.
And what the excessive appetite of some is in lawful food, this that intercourse that is matter of pardon is in husband and wife. As therefore it is better to die of hunger than to eat things offered unto idols: so it is better to die without children, than to seek a family from unlawful intercourse. But from whatever source men be born, if they follow not the vices of their parents, and worship God aright, they shall be honest and safe. For the seed of man, from out what kind of man soever, is the creation of God, and it shall fare ill with those who use it ill, yet shall not, itself at any time be evil.
But as the good sons of adulterers are no defense of adulteries, so the evil sons of married persons are no charge against marriage. Wherefore as the Fathers of the time of the New Testament taking food from the duty of conservation, although they took it with natural delight of the flesh, were yet in no way compared with the delight of those who fed on what had been offered in sacrifice, or of those who, although the food was lawful, yet took it to excess: so the Fathers of the time of the Old Testament from the duty of conservation used sexual intercourse; and yet that their natural delight, by no means relaxed unto unreasonable and unlawful lust, is not to be compared either with the vileness of fornications, or with the intemperance of married persons.
Forsooth through the same vein of charity, now after the spirit, then after the flesh, it was a duty to beget sons for the sake of that mother Jerusalem: but it was nought save the difference of times which made the works of the fathers different. But thus it was necessary that even Prophets, not living after the flesh, should come together after the flesh; even as it was necessary that Apostles also, not living after the flesh, should eat food after the flesh.
Source: On the Good of Marriage (New Advent)