30 Referring, then, to the eternal law which enjoins the preservation of natural order and forbids the breach of it, let us see how our father Abraham sinned, that is, how he broke this law, in the things which Faustus has charged him with as highly criminal. In his irrational craving to have children, says Faustus, and not believing God, who promised that his wife Sara should have a son, he defiled himself with a mistress. But here Faustus, in his irrational desire to find fault, both discloses the impiety of his heresy, and in his error and ignorance praises Abraham's intercourse with the handmaid.
For as the eternal law— that is, the will of God the Creator of all— for the preservation of the natural order, permits the indulgence of the bodily appetite under the guidance of reason in sexual intercourse, not for the gratification of passion, but for the continuance of the race through the procreation of children; so, on the contrary, the unrighteous law of the Manichæans, in order to prevent their god, whom they bewail as confined in all seeds, from suffering still closer confinement in the womb, requires married people not on any account to have children, their great desire being to liberate their god.
Instead, therefore, of an irrational craving in Abraham to have children, we find in Manichæus an irrational fancy against having children. So the one preserved the natural order by seeking in marriage only the production of a child; while the other, influenced by his heretical notions, thought no evil could be greater than the confinement of his god.
Source: Reply to Faustus the Manichaean (New Advent)