On June 26, 2013, the United States Supreme Court issued two milestone decisions pertaining to same-sex marriage.
The first decision struck down DOMA (The Defense of Marriage Act of 1996) as a violation of the 5th Amendment. One of the major provisions of DOMA is that it denied federal benefits to same-sex married couples in states that have such legal marriages.
The second decision, in essence, allowed a lower court's overturning of California's Proposition 8 to stand. Prop 8 was a 2008 ballot initiative passed by California voters. It banned same-sex marriage in California. But the measure was struck down by a court, and the government of the State of California refused to defend Prop 8. The Supreme Court essentially determined that the defenders of Prop 8 did not have the legal standing to defend the ballot initiative. As such, Proposition 8 no longer has legal standing in California. It appears that state-sanctioned same-sex marriages will resume shortly in the most populated state in the U.S.
One of the most basic consequences stemming from the Court's decision on Proposition 8 is that a significantly larger portion of the U.S. population will now be living in an area where same-sex marriage is legal. California has a population of around 38 million, about 12% of the total U.S. population.
The long-run cultural and political ramifications of these decisions are impossible to predict with great precision, but it stands to reason that certain aspects this deeply divisive issue have been further tilted to the advantage of the proponents of same-sex marriage.
UPDATE: As the day progressed, the legal ramifications of the Prop 8 decision became a bit more cloudy. It appears that the State of California will resume the granting of same-sex marriage licenses in about a month. Some political conservatives indicated, however, that new legal challenges might be made in California.
Time.com: The SCOTUS's Historic Decisions on Same-Sex Marriage
Washington Post: The SCOTUS Strikes Down DOMA