61 Judah, as Faustus says, committed fornication; and besides that, we can accuse him of selling his brother into Egypt. Is it any disparagement to light, that in revealing all things it discloses what is unsightly? So neither is the character of Scripture affected by the evil deeds of which we are informed by the record itself. Undoubtedly, by the eternal law, which requires the preservation of natural order, and forbids the transgression of it, conjugal intercourse should take place only for the procreation of children, and after the celebration of marriage, so as to maintain the bond of peace.
Therefore, the prostitution of women, merely for the gratification of sinful passion, is condemned by the divine and eternal law. To purchase the degradation of another, disgraces the purchaser; so that, though the sin would have been greater if Judah had knowingly lain with his daughter-in-law (for if, as the Lord says, man and wife are no more two, but one flesh, a daughter-in-law is the same as a daughter); still, it is plain that, as regards his own intention, he was disgraced by his intercourse with an harlot.
The woman, on the other hand, who deceived her father-in-law, sinned not from wantonness, or because she loved the gains of iniquity, but from her desire to have children of this particular family. So, being disappointed in two of the brothers, and not obtaining the third, she succeeded by craft in getting a child by their father; and the reward which she got was kept, not as an ornament, but as a pledge. It would certainly have been better to have remained childless than to become a mother without marriage.
Still, her desire to have her father-in-law as the father of her children was very different from having a criminal affection for him. And when, by his order, she was brought out to be killed, on her producing the staff and necklace and ring, saying that the father of the child was the man who had given her those pledges, Judah acknowledged them, and said, "She has been more righteous than I"— not praising her, but condemning himself. He blamed her desire to have children less than his own unlawful passion, which had led him to one whom he thought to be an harlot.
In a similar sense, it is said of some that they justified Sodom; that is, their sin was so great, that Sodom seemed righteous in comparison. And even allowing that this woman is not spoken of as comparatively less guilty, but is actually praised by her father-in-law, while, on account of her not observing the established rites of marriage, she is a criminal in the eye of the eternal law of right, which forbids the transgression of natural order, both as regards the body, and first and chiefly as regards the mind, what wonder though one sinner should praise another?
Source: Reply to Faustus the Manichaean (New Advent)