Lookups spiked on June 25, 2011.
New York became the sixth state to legalize same-sex marriage.
Marriage has become a hot button 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 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.