In the same-sex marriage cases both sides have enlisted foreign law in support of their position. A group of scholars led by Harold Koh argues that the recent embrace of same-sex marriage in the law of other Western nations supports finding a right to same-sex marriage in our own Constitution. Another group of scholars, led by Lynn Wardle, observes that many more nations, including in some cases their judiciaries, have declined to interpret their guarantees of equality to mandate same-sex marriage.
Originalists must generally reject the evidence of contemporary foreign law in constitutional interpretation. Such laws should not be used as authority or indeed given any weight in decision making not because they are foreign or international, but because they are contemporary and in the ordinary case shed no light on the original understanding of the Constitution. Originalists would object to contemporary American sources for the same reasons. In contrast, considering foreign or international law from the time a provision was framed could potentially advance our understanding of the original meaning of the Constitution. Rejecting foreign law is not an act of xenophobia, but a reflection of appropriate interpretive formalism.
But for more directly pragmatic reasons, even those who are not originalists should not use foreign law to cast doubt either on the constitutionality of our own law or on the otherwise correct interpretation of the Constitution. Foreign law inherently evolves alongside the norms of a nation other than our own. A foreign law, including a foreign court decision, is also simply not framed with reference to being applied anywhere but to its own nation. Even where that law is the product of a democratic consensus or a judicial interpretation of a constitutional consensus, it remains the consensus of citizens of another nation. Any conflict between that law and our own thus does not suggest on its face that our law is not beneficial as applied here. In this sense, reliance on foreign law is an even less plausible basis to displace our own constitutional norms than reliance on international law, because the latter at least purports to possess universal scope.
Moreover, any particular foreign law is part of a complex system of norm creation. For instance, the United States has a structure of federalism and more general traditions of decentralization that are important processes for testing the content of rights. Many European nations are far more centralized and thus same-sex marriage could not gain impetus, as it has in the United States, from decisions at the subnational level. That difference has an obvious pragmatic relevance to whether a national court should intervene. Thus, foreign constitutional norms do not just reflect certain views about substantive rights; they also reflect a foreign mode of defining them.
In short, any law from another culture is the culmination of a complex institutional structure of producing norms. Any foreign law on same-sex marriage is the surface manifestation of a far deeper social structure that is likely foreign to America.