Lookups spiked on March 26, 2013.
The word became the subject of renewed scrutiny as the Supreme Court heard arguments in cases seeking to overturn California's ban on gay marriage and the federal government's Defense of Marriage Act.
Marriage has become a controversial definition, although its original sense – "the state of being united to a person of the opposite sex" – has not changed.
However, because the word is used in phrases such as "same-sex marriage" and "gay marriage" (by proponents and opponents alike), a second definition – "the state of being united to a person of the same sex in a relationship like that of a traditional marriage" – was added to the dictionary to provide an accurate picture of the word's current use.
Here's more information about how words get into the dictionary.