Final answer:
Same-sex marriage became legal in the United States in 2015 through a Supreme Court decision in the case Obergefell v. Hodges, following earlier cases and decisions that laid the groundwork for legalization.
Step-by-step explanation:
The US Supreme Court legalized same-sex marriage throughout the United States on June 26, 2015, in the landmark case Obergefell v. Hodges. Prior to this, several court cases and decisions laid the groundwork for legalized same-sex marriage in different states. For example, the 2013 decision in United States v. Windsor overturned part of the Defense of Marriage Act (DOMA), while the Supreme Court's dismissal of Hollingsworth v. Perry in 2014 affirmed the unconstitutionality of California's Proposition 8. By 2015, more than half of the states had already legalized same-sex marriage.