In a 5-4 ruling in United States v. Windsor, the court struck down a provision of the 17-year-old Defense of Marriage Act (DOMA) that denies federal benefits -- like Social Security benefits or the ability to file joint tax returns -- to same-sex couples legally married.

"DOMA is unconstitutional as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment," Justice Anthony Kennedy wrote for the majority. Kennedy was joined by Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan.

At the same time, the court ruled 5-4 that the defendants in the case ofHollingsworth v. Perry, which considered the constitutionality of California's same-sex marriage ban (called Proposition 8), have no standing in court. Supporters of Prop. 8 brought the case to the Supreme Court after a lower court struck down the law but California's governor and attorney general declined to defend it. By dismissing the case on procedural grounds, the court passed up the opportunity to issue a significant ruling on the issue of marriage.