It may be a little late to the game to be talking about this, now that a clear majority of people in Canada and the U.S. actually favour same-sex marriage, but just the other day I was talking to someone who brought up the linguistic argument. He had no problem whatsoever with same-sex relationships; he just objected to changing the meaning of the word "marriage", which (he argued) has always been understood to mean a particular sort of relationship between a man and a woman, and that if we're going to sanction similar sorts of relationships between men and men, or women and women, we should come up with a new word for it, rather than dilute the meaning of good old-fashioned "marriage".
This seems like the sort of argument that would appeal to someone like me, because I tend to be the grammar/usage purist at dinner parties and similar events, and I place a great deal of value on using words correctly. I reliably object whenever someone tries to explain acupuncture or shiatsu using phrases like "energy lines", because energy is a well-defined and quantifiable scientific concept: force times distance. I tremble with rage at "irregardless" and don't even get me started on using quotation marks for "emphasis".
And yet, expanding the set of relationships captured by the word "marriage" does not bother me at all. Why?
I don't love words and grammar and punctuation rules for their own sake. All by themselves, they're kind of arbitrary, and any number of other equally effective rules could be devised. Indeed, they have been: that's what other languages are. In English, we tend to distinguish between subject and object by word order (subject - verb - object), but in Japanese, the subject is often simply implied, the object marked by a particle, and the verb at the end of the sentence goes.
No, I care about English words and the rules of grammar because I understand how versatile they are and how they can skillfully be used to convey meanings with rigorous precision or with playful ambiguity. I don't object to people knowingly misusing a word for effect; that's not actually a misuse. I do object to habitual misuse that degrades a useful meaning so that I can't use it anymore, and have to go into a long pedantic exposition before I can get to my main point. (Okay, so maybe I seem to like being pedantic, but I'd rather be able to get to the main point. When I have one, anyway.)
So I want the words I use to be useful. I want them to capture the meaning that is really at the core of what I'm talking about, and not merely some label for an arbitrary list of elements. Let's imagine, for example, that everybody only ever used a Thermos to keep hot drinks hot, and if you asked anyone what a Thermos was, they'd say "It's a special kind of bottle that keeps hot things hot." Let's say the word came to be defined that way (assuming it lost its trademark status, that is), and dictionaries universally adopted that definition.
Then, someone discovers, that if you put a cold liquid in a Thermos, it stays cold longer! My goodness! What a discovery! Whatever shall we call this new function? If a Thermos is a device that keeps hot things hot, we can't call it that. Maybe Cryos or something?
But that's silly. It's the same object, whether it's used to keep things hot or keep things cold, and moreover its actual function is neither, but to limit the flow of heat energy between the inside and the outside, whatever the temperature is. The appropriate thing to do is to revise our definition of the word, not to invent some brand new one in order to preserve an outdated (mis)understanding of what the old thing was.
I applaud this kind of refinement of meaning. Words are tools, and I want them to be the best tools we can make them. We should be wary of discarding their traditional meanings too quickly, because very often there are good reasons for why a word came to mean what it does; the words we have today are the product of many generations of productive bickering among writers and speakers and philologists who probably raised and considered many of the same concerns we think we're bringing up new today. But we should also be willing to change the old meaning when it is clearly inferior to the new proposal.
And that's what I think is true of the word "marriage" today. I don't necessarily accept that the word itself just meant one man and one woman before, but even if I did, I submit that that's a pretty inelegant kind of word to preserve. We know now that the legal status that goes by the name of "marriage" when it applies to a man and a woman can also perform the same function with a man and a man or a woman and a woman. Insisting that we come up with a new word for it in those cases is just as silly as insisting that we can't call something a Thermos when it's used to keep something cold instead of hot.
Yes, I care deeply about the integrity of our language, and to me, the word "marriage" is made stronger and more useful, not diluted, by expanding it to include all spousal relationships regardless of the gender permutations involved. What that couple has is a marriage, and I don't actually need to know what their genders are in order to understand the essential qualities of their relationship.