Louisiana law bars marriage between first cousins — but it also provides that foreign marriages should be recognized, even if they would otherwise be illegal, unless they violate "a strong public policy" of the state. This is indeed a traditional rule recognized in many states.

And it makes sense: Marriage is supposed to help people organize their lives around their marital status, and generally speaking that sort of self-organization is helpful to society more broadly (which is why the law specially recognizes and in various ways tries to encourage marriage). If people got married in a foreign jurisdiction, treating them as unmarried in the place in which they move will frustrate their expectations, likely harm one or both parties, and likely cause some harm to the state's interests that marriage advances. A state normally wouldn't want this to happen, unless it's condemnation of this particular marriage is very strong indeed.

In any case, it's clear that Louisiana law recognizes three categories of marriage: (1) A clearly defined category of marriages that may be entered into in Louisiana; (2) a less well-defined category of marriages that may not be entered into in Louisiana but that are recognized if they were validly entered into in another state or country; and (3) a correspondingly not very well-defined category of marriages that aren't recognized in Louisiana at all. The question in Ghassemi v. Ghassemi was whether first cousin marriages fall into category 2 or category 3. And to answer that it is of course not enough to look at the specific details set forth in Louisiana marriage rules (which generally defines category 1 marriages, and doesn't speak in detail to the boundary between category 2 and category 3). One also has to engage in the vaguer inquiry of whether the public policy against first cousin marriage — indubitably a policy that Louisiana law does recognize — is "strong" enough to overcome the general presumption of recognizing even those out-of-jurisdiction marriages that couldn't be legally entered into in Louisiana.

In measuring the "strength" of this policy, the Court of Appeal (in an opinion by Republican Judge Jimmy Kuhn) looked, first, to whether Louisiana law categorically prohibits all first-cousin marriages and sexual relationships; it concluded that there have long been various exceptions to this prohibition. (The court specifically notes "In so concluding we note that the Louisiana Legislature has not expressly outlawed marriages between first cousins regardless of where they are contracted as it has emphatically done in the case of purported same sex marriages" (emphasis in original).) But then, it also looked to various other sources as to the depth of the prohibition on first cousin marriage, including:

1. "natural law" (which Louisiana courts seem to refer to much more often than do other state courts, perhaps because of Louisiana's civil law tradition; the court cited an old American Law Reports annotation that discussed this subject),

2. "Bible's Book of Leviticus, the font of Western incest laws,"

3. the views of other states (of which about half allow some or all first-cousin marriages),

4. the views of other "western countries": "the U.S. is unique among western countries in restricting first cousin marriages."

This strikes me as a pretty reasonable of evaluating the strength of a state's public policy in this particular situation. By definition, we've got a question that under state statutes has no precisely defined answer; the statutes expressly call for courts to determine how "strong" the public policy against the marriage is. The court should then look to other clues to the state's public policy. First, it should see what can be gleaned from other rules within the same state. Second, it should look at those rules that are historically the sources of the state's legal system (and on this point the Bible strikes me as relevant, not because of its religious nature but because of its historical role in shaping European and American incest law). And third, it should look at what similar jurisdictions think about the subject, starting with other states of the same nation but going on to other nations within the same broad culture.

Now I generally agree that American courts generally shouldn't rely on modern foreign law in interpreting the meaning of the U.S. Constitution. It's true that Western countries are often trying to solve similar problems with their constitutions, but American constitutional practices have their own history, rest on their own text, and by design created a constitutional regime that is supposed to be individualized to American life and American political thought.

But I think it's a mistake to overgeneralize this into a broad hostility to the "use of foreign law" in American courts. Using foreign law even to determine the scope of domestic legal principles is often quite sensible, even if not to determine the substantive scope of American constitutional rules — as even Justice Scalia has agreed.