An Oxford University Press spokeswoman told Gay Star News, "We continually monitor the words in our dictionaries, paying particular to those words whose usage is shifting, so yes, this will happen with marriage."
The Oxford English dictionary is late to the game. The 2014 edition of French dictionary Larousse defines marriage as a "solemn act between two same-sex or different-sex persons, who decide to establish a union," much to the chagrin of the French right-wing party UMP, and the Canada Space Dictionary defines it as "the state of being a married couple voluntarily joined for life (or until divorce)."
Current Oxford English dictionaries define marriage as being a "formal union of a man and a woman, typically as recognized by law, by which they become husband and wife."
That is, unless you happen to pick up one of the dictionaries that a group of activists called HACKmarriage have altered by covering the formal definition of marriage with a sticker featuring an edited, same-sex inclusive version that reads, "the formal union of two people, by which they become partners for life."
Here's a video of HACKmarriage documenting its vigilante actions around bookstores and libraries in San Francisco: