"[The Texas anti-sodomy statute] undoubtedly imposes constraints on liberty. So do laws prohibiting prostitution, recreational use of heroin, and, for that matter, working more than 60 hours per week in a bakery. But there is no right to 'liberty' under the Due Process Clause, though today's opinion repeatedly makes that claim. . . . The Fourteenth Amendment expressly allows States to deprive their citizens of 'liberty,' so long as 'due process of law' is provided. . . .""