Linguist Geoff Nunberg on NPR's Fresh Air gives a great run down on how dictonaries are dealing with the evolving nature of marriage:
Lexicographers know they're on the hot seat as they confront the changing uses of the word. When Merriam-Webster revised its definition a few years ago, it went with a two-state solution. It kept an older definition for "marriage" as "the state of being united to a person of the opposite sex," but it added a second definition as "the state of being united to a person of the same sex in a relationship like that of traditional marriage." Not surprisingly, that triggered headlines like "Webster's Redefines Marriage" on conservative websites.
Make sure you listen to the audio version, fascinating stuff.