Actually, it's not. If the public acts and judgments of the States were not accorded full faith and credit in the other States, we would be the European Union, not a federal Republic.
Actually we are. Not enough attention is being given the the three European Court of Human Justice precedents cited in the majority opinion on the Texas case. While the actual way in which these European precedents were used is a little fuzzy, the way I read it is this: The 14th amendment guarantees Americans liberty, and the European Court, combined with whatever US public opinion will more or less stomach, defines what liberty is.
Creating a precedent for our law being determined by foreign courts was simply too high a price to pay for a very few Texas gays not being subject to misapplied prosecutorial discretion.