This is not quite true. Have you read "Lawrence v. Texas"?
Anyway, the "Lawrence" decision was written by Anthony Kennedy and is a good decision written by a Justice who is quite mediocre and mostly lets his clerks do the writing. I have read the decision several times and don't see the postmodern concepts you claim.
The decision is here. http://www.law.cornell.edu/supct/html/02-102.ZO.html
But to claim that the usual obfuscations of the law (a premier source of ideological mystification in capitalist society) justify the less important obfuscations of postmodernism (a minor source of ideological mystification in our intellectual culture) does not seem to me a good way to argue for postmodernism. But this goes to the roots of previous political choices. How do you fight ideological mystification? This I think should be the source of our arguments over these issues.
There is a more complicated question that I think you are getting at here. What are the influence of larger social movements on the law. Scalia claims in his dissent in "Lawrence" that the Supreme Court majority in this case is mainly responding to the elite culture of law schools and law professors. In other words he makes a claim something like your claim that postmodernism is an influence on the court majority. This doesn't make your claim or Scalia's wrong, but I don't buy the social analysis implicit in the claim.
In order to look at the change from "Bowers v. Hardwick" to "Lawrence v. Texas" I would advise looking at larger social movements in our society and not to the floating influence of "postmodern concepts."
Amusingly, there are also claims that Scalia's philosophy and his dissent in Lawrence is very similar to Foucault's philosophy. There are law review articles on this subject if you wish to be amused I will give you the references. If you wish to be bored you can actually read them. Such cogitations are mostly about how law professors and related academics must write something/anything, in order to make their trek through the academy.
For example:
"Nietzsche, Foucault, Scalia"
Emory Public Law Research Paper No. 06-28
Full Text: http://ssrn.com/abstract=948311
ABSTRACT: This paper explores the narrative strategies of majority and dissenting opinions in Lawrence v. Texas, Romer v. Evans, and Bowers v. Hardwick, all major lesbian/gay civil rights decisions. It demonstrates that the story of U.S. history - increasing protection for individual rights, or decreasing respect for moral and constitutional tradition - explains as much about the legal outcome as the doctrinal arguments that the opinions contain. In particular, it places these opinions into a discussion about the relationship between narrative and identity, individual and national. From this perspective, Justice Antonin Scalia shares with French philosopher Michel Foucault the belief that narrative is closely related to identity, with the important difference that Foucault celebrates the fragility of this connection while Scalia deplores it.
Now I read this paper and I find it interesting and I think that the point about the outcome not being explained by "doctrinal arguments" is quite true. I don't buy the analysis of "narrative and identity". Mostly you have to accept the whole "problematic" of narrative and identity in the first place in order to accept the argument. In my view the larger social movements do not have to be placed into the boundaries of narrative and identity, though if that is your tendency of analysis then you might buy it.
Jerry Monaco