I feel I should point out that the Oxford and webster dictionaries actually define marriage as a union between a "husband" and "wife", and then go on to specifically call a "husband" and "wife" as a "man or woman (respectively) in the state of marriage or matrimony"
for quick reference you can look here:
http://dictionary.reference.com/browse/marriage?s=t
Take notice of the fact that it includes a sub-term "gay marriage" as a "similar institution" between 2 people of the same sex. Most people don't realize this, but in most dictionaries, contexts, and societies, Marriage is in fact defined as heterosexual in nature (That is not to say that widespead opinion shouldnt change this fact).
In terms of the origins of the word "Marriage", It was created between 1250–1300 AD during the middle ages as referenced to the Middle English
mariage of different spelling.
So actually Marriage has always been between a Man and a Woman and is in many circles considered a type of slang when used with same sex relationships. I'm not putting a judgement here, I am simply stating why I think we all get hung up on the idea of changing the meaning of the word Marriage when we really should just stop using that term to describe something it traditionally isnt (Even if we want it to). It is a bit like deciding you are just going to start calling a hamburger a hot dog because they both contain meat.
What we should really do is come up with a term, other than marriage, that best describes a relationship between two individuals based on love (since that is what people defend as their need to call it a marriage). I mean honestly, there is nothing in the definition of marriage that says ANYTHING about love and if you know anything about history it has CERTAINLY been an optional component.