United States v. Windsor
[Page 14 of Opinion]
Slowly at first and then in rapid course, the laws of New York..recognized same-sex marriages...
After a statewide deliberative process that enabled its citizens to discuss and weigh arguments for and against same-sex marriage, New York acted to enlarge the definition of marriage
...Against this background of lawful same-sex marriage in some States...By history and tradition
the definition and regulation of marriage, as will be discussed in more detail, has been treated as being within the authority and realm of the separate States...
[page 16]
..it is necessary to discuss the extent of the state power and authority over marriage as a matter of history and tradition. State laws defining and regulating marriage, of course, must respect the constitutional rights of persons, see , e.g., Loving v.Virginia, 388 U. S. 1 (1967); but, subject to those guarantees, “regulation of domestic relations” is“ an area that has long been regarded as a virtually exclusive province of the States.”...
[page 17]
The recognition of civil marriages is central to state domestic relations law applicable to its residents and citizens. See Williams v.North Carolina, 317 U. S. 287,298 (1942) (“Each state as a sovereign has a rightful and legitimate concern in the marital status of persons domiciled within its borders”

. The definition of marriage is the foundation of the State’s broader authority to regulate the subject of domestic relations with respect to the“[p]rotection of offspring, property interests, and the enforcement of marital responsibilities.” Ibid.
“[T]he states, at the time of the adoption of the Constitution, possessed full power over the subject of marriage and divorce. . . [and] the Constitution delegated no authority to the Government of the United States on the subject of marriage and divorce.”