There has been some brief discussion of the use of international law in U.S. courts over at the Volokh Conspiracy and Opinio Juris. My view of the issue is that there are very few people who are actually "internationalists" here. I see three general groups out there:
(1) Those who cite international law when it supports their views and ignore it where it does not
(2) Those who think international law should either trump domestic law or play an important role in its interpretation and formation, regardless of whether they agree with the substance
( 3) Those who think international law should play at most a minor role in the interpretation and formation of domestic law, regardless of whether they agree with the substance
Most of what I read, in both the popular press and from legal experts, falls into (1). People in (2) are true internationalists who believe, for various reasons, that the international is inherently good. People in (3) are true sovereigntists who believe, for various reasons, that the international is inherently bad. But people in (1) really only care about the particular issue they are interested in (e.g. prohibiting gay marriage or allowing it) and cite to international/foreign sources of law that support their view. They don't care about the proper role of internatinoal law in U.S. law. Thus, to me, they are only "internationalists of convenience."