30 But why do these persons think they may imitate Tamar telling a lie, and not think they may imitate Judah committing fornication? For there they have read both, and nought of these has that Scripture either blamed or praised, but has merely narrated both, and to our judgment dismissed both: but it is marvellous if it has permitted anything of these to be imitated with impunity. For, that Tamar not through lust of playing the harlot, but through wish of conceiving seed, did tell the lie, we know.
But fornication also, howbeit Judah's was not such, yet some man's may be such whereby to procure that a man may be delivered, just as her lie was in order that a man might be conceived; is it right then to commit fornication on this account, if on that account it is thought that it was right to lie? Not therefore concerning lying only, but concerning all works of men in which there arise as it were compensative sins, must we consider what sentence we ought to pass; lest we open a way not only to small sins whatsoever, but even to all wickednesses, and there remain no outrageous, flagitious, sacrilegious deed, in which there may not arise a cause upon which it may rightly seem a thing meet to be done, and so universal probity of life be by that opinion subverted.
Source: To Consentius: Against Lying (New Advent)