One of the arguments that anti-gay marriage people always use is that we shouldn't redefine marriage. It's as if they don't really care about marriage or gay people, they're just worried about the integrity of the dictionary.
The argument is a bad one anyway. There is no such thing as "the" definition of marriage. The words marry and marriage have multiple meanings. Marry and marriage have always had multiple meanings. Some of those meanings specify a man and a woman. Some of them specify a man and women. Some of them say nothing about gender. The earliest usage of the word dates to 1297 when it was defined as "to wed, marry, give in marriage."
So, next time you hear this lame argument, ask the person spouting it, "which definition"? That's what it's really about, choosing which definition is acceptable and which is not. It isn't about redefining anything.