Thomas believes that his own position on this particular law, and law in general, is principled and consistent.
Law is meant to conform humans to a standard that preserves the cultural and moral order. The purpose of government is to “secure” unalienable pre-existing rights about which Thomas Jefferson wrote in the Declaration of Independence...Government is not supposed to create new rights like national health care, or same-sex marriage.
I actually admire Thomas's restraint in not dragging the "Creator" into this, as he'd usually do. The natural-law tradition he invokes here, however, is bad enough whether it invokes God or not. It's also impossible to verify. Since not even the most devout theocrats claim that we've enjoyed all our natural rights from the beginning of civilization, the natural-law or natural-right tradition has been a process of gradual discovery. How can anyone claim that we've now reached the limit of our natural rights? How can they know, and how can their limitation be trusted when every previous assertion of "natural" right has been challenged on the exact same ground? How do we know that arguments against same-sex marriage, for instance, aren't just bigoted?
Thomas expects such a question. He's resigned to having "the equivalent standing of 1950s segregationists" among liberals for opposing same-sex marriage. In their view, he claims, "Anyone arguing for tradition is branded a bigot, a label that is supposed to end all discussion." This isn't quite accurate. Anyone arguing for a bigoted tradition is rightly branded a bigot, and the opponents of gay rights in the 21st century are the moral equivalent of the racists and anti-semites of earlier times. Bigotry is not redeemed or sanctified by age. Two-thousand year old bigotry is still bigotry. Are Pashtuns not bigoted against women because their tradition tells them to treat women that way? It is likewise with homophobes everywhere. The "word of God" doesn't transmute bigotry into principle; it only proves someone a superstitious bigot.
Still, it's a fair question in a democratic republic whether the people have the power or right to dictate the terms of marriage. Obviously, people can claim the right to forbid same-sex or plural marriage, but are they justified by any compelling public interest in the sexual relations or shared property of two or more people? Thomas argues that the Supreme Court denied rights to polygamists on "general welfare" grounds. But who defines the general welfare? Did the Founders do so for all time in 1787? Is it up to a popular vote at the drop of a hat? I'd like to read the Reynolds v. United States ruling that Thomas cites to see how polygamy influenced the general welfare in those judges' opinion. I suspect that their definition of general welfare wouldn't stand the test of time, but who can say whether mine or ours would, either? To the extent that marriage involves reproduction or the raising of children, general welfare might well be involved, but conventional heterosexual marriage might well be subject to fresh regulation on similar grounds, whether husbands and wives want it or not. Leaving aside the self-evident absurdities of tradition of religion, the definition of marriage in a democratic society is arguably part of how citizens affirm their accountability to one another, and thus always a subject for political debate, no matter how much anyone would want to exclude it from debate as an eternal absolute right. As I see it, it's people like Thomas who try to place tradition above criticism who try to "end all discussion," not those who challenge it in the media, in the courts, or on the street.