The basic point of the passage from Cicero’s Laws cited here the other day reappears in the Westminster Confession of Faith (of all places) in the chapter on–coincidentally enough–marriage.
Treating incestuous marriages, the Divines say:
IV. Marriage ought not to be within the degrees of consanguinity or affinity forbidden by the Word. Nor can such incestuous marriages ever be made lawful by any law of man or consent of parties, so as those persons may live together as man and wife. The man may not marry any of his wife’s kindred, nearer in blood then he may of his own: nor the woman of her husband’s kindred, nearer in blood than of her own.
Where Cicero speaks of the higher law to which all positive law is accountable as the law of nature, the WCF here speaks of it as the Word. It is of course interesting, from our modern vantage-point, to note that the Divines considered the principles of the Word to be normative for civil law. But we should not, I think, get hung up on this point, for the mainstream of Christian theologians for centuries would have considered the moral law as explicated in Scripture to be harmonious with the law of nature–indeed, as its summary–because the author of both is God; and the civil prescription in this chapter, as set forth in the Word, is based, one may safely assume, upon moral principle. It stands to reason, then, that incestuous marriages violate not only the Word, but nature itself.
What is significant is the reiteration, yet again, that simply making laws about certain actions or modes of relation does not make them lawful stricto sensu. If they do not answer to something metaphysically superior and more foundational, such laws are merely a trick of language with no ontological heft. I could, for instance, call my 2002 Nissan Maxima a 1950 Jaguar XK120. But I’m fairly certain I know what I’ll find in my driveway when I walk out the door tomorrow morning.