It all stems from this: "No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws." Shorn of all rhetoric and legal technicality, it comes down to these two interlocking syllogisms:
Syllogism No. 1.
1. The Constitution protects fundamental rights
2. The right to marry is a fundamental right.
3. The Constitution protects the right to marry.
Syllogism No. 2.
1. A strong, constitutionally permissible reason must be articulated to deny people a fundamental right.
2. The only reasons articulated by anti-same sex marriage advocates--"it's against my religion" and "tradition"--are not constitutionally permissible grounds, the first violating the Establishment Clause and the second inconsistent with our system of case by case adjudication.
3. The bans against SSM cannot stand.
Now, since the dissents harp on tradition, let me address that prong at a little greater length. As a wiser judge than any of the dissenters in Obergefell wrote many years ago:
History must be a part of the study, because without it we cannot know the precise scope of rules which it is our business to know. It is a part of the rational study, because it is the first step toward an enlightened scepticism, that is, towards a deliberate reconsideration of the worth of those rules. When you get the dragon out of his cave on to the plain and in the daylight, you can count his teeth and claws, and see just what is his strength. But to get him out is only the first step. The next is either to kill him, or to tame him and make him a useful animal. For the rational study of the law the blackletter man may be the man of the present, but the man of the future is the man of statistics and the master of economics. It is revolting to have no better reason for a rule of law than that so it was laid down in the time of Henry IV. It is still more revolting if the grounds upon which it was laid down have vanished long since, and the rule simply persists from blind imitation of the past.This is the case here. Because when Chief Justice Roberts--in the least offensive of the dissents, one that at least tries to do actual legal reasoning--adverts to the tradition of marriage, he writes:
The fundamental right to marry does not include a right to make a State change its definition of marriage. And a State’s decision to maintain the meaning of marriage that has persisted in every culture throughout human history can hardly be called irrational.It's important to note that, in this brief "historical" allusion, he's not following Justice Holmes's advice and bringing the dragon out of his cave at all; he's assuming that all marriage was, in every culture, the same, ignoring the fundamental changes that have been brought to marriage. More bluntly, Roberts is retrojecting his nice, loving marriage, in which each spouse has a right to seek fulfillment in a career into past decades, centuries, and millenia.
In the Framer's generation, Sir William Blackstone, in his Commentaries wrote that "By marriage, the husband and wife are one person in law: that is, the very being or legal existence of the woman is suspended during the marriage, or at least is incorporated and consolidated into that of the husband . . . For this reason, a man cannot grant anything to his wife, or enter into covenant with her: for the grant would be to suppose her separate existence; and to covenant with her, would be only to covenant with himself," making contracts between husband and wife unenforceable. Blackstone's Commentaries were highly influential in early American law; thus in the 1848 Seneca Falls Declaration, a cause of complaint of women was that man has "made her, if married, in the eye of the law, civilly dead." (More and sources here).
Throughout the 19th Century, a series of state laws chipped away at the inability of married women to own property, fundamentally changing the nature of marriage.
Throughout the Twentieth Century, states began to jettison the 17th Century rule that husbands could not be guilty of raping their wives, "for by their mutual matrimonial consent and contract, the wife [has] given up herself in this kind unto her husband, which she cannot retract." The last one fell in 1993.
I could go on--catalogue the societies in which polygamy has been acceptable, concubinage permitted, and dilate upon the difference between arranged and companionate marriage. All true, all blandly swept into the corner by Roberts.
The fact is, marriage has not remained static. It has grown and evolved and changed fundamentally already. This is just sharing the modern marriage the Robertses enjoy with a class of people who have been excluded until recently, based on theological readings of the Bible that are rooted in a weak and selective example of traditionalist thinking that offshores the cost of obedience on a disfavored minority class--just when Supreme Court review is most called for.
In sum, this was the Court at its best, doing its job.
[Edited to Note Sir Matthew Hale was a 17th Century jurist, not an 18th century one.