I guess dictionaries are somewhat like laws.  Often they just document what is already happening in society.

Here’s one of the latest — a definition of "marriage," from the Online Dictionary of Miriam Webster.

Main Entry:
mar·riage 
Pronunciation:
\?mer-ij, ?ma-rij\
Function:
noun
Etymology:
Middle English mariage, from Anglo-French, from marier to marry
Date:
14th century

1 a (1): the state of being united to a person of the opposite sex as husband or wife in a consensual and contractual relationship recognized by law (2): the state of being united to a person of the same sex in a relationship like that of a traditional marriage <same-sex marriage> b: the mutual relation of married persons : wedlock c: the institution whereby individuals are joined in a marriage

2: an act of marrying or the rite by which the married status is effected ; especially : the wedding ceremony and attendant festivities or formalities

3: an intimate or close union <the marriage of painting and poetry — J. T. Shawcross>

So, we can have debates over whether there is legal or moral precedence for same-sex marriage, but at this point there is a now officially a definition (although somewhat ambiguous by saying it is "like a traditional marriage" — got to love those adjectives!!).