According to various folks opposed to same-sex marriage, the definition of marriage has been a constant forever. The idea of changing what marriage is should be (they say) anathema.
I'm sure they'll be pleased to read an interview with Susan Squire about the history of marriage.
For example, I imagine most people who believe marriage has never changed will be glad to learn this about marriage in Western Europe a few hundred years ago:
Especially in the aristocracy, you didn't marry the person you wanted to marry. That had nothing to do with it. You bred with the person who could bring peace to the land, or bring you more feudal servants, or enhance your bloodline.
Just like today! See, marriage hasn't changed a bit.
In fact, it hasn't changed since Ancient Greece:
The average age of male marriage in Athens was about 25 to 30, and they usually took a wife of about 15. [...] The Athenian wives were not allowed out of the house because they would be seen by other men, and that might cause them to stray. It was considered unseemly for men to love women--either their wives or, more likely, their courtesans--because women weren't worthy of the highest form of love. The highest form of love was cerebral love between men.
Again, just like today. It's remarkable how constant and unchanging marriage has been for thousands of years.
Of course, most of the abovequoted material doesn't explicitly say who can and can't get married. So let's look at that.
Wikipedia's discussion of the history of marriage is full of "citation needed"s, so I won't quote much of it, but from the parts that do have cites, we see even more evidence of the pure constancy of marriage:
In the 1200s in England it was unlawful for a woman younger than 24 years to marry.
Also, prior to 1907, in the UK, Wikipedia explains:
[I]t was forbidden for a man to marry the sister of his deceased wife.
Once again, just like today--as we all know, it is contrary to the laws of God and man, everywhere in the universe, and always has been, for a widower to marry his dead wife's sister.
Speaking of geographical universality: as we can see just by looking at a map, right here in these United States, cousin marriage is and has always been completely illegal everywhere in the world.
And, yeah, interracial marriage is still illegal in the US, just like it was fifty years ago.
I don't think I even need to mention the monogamy thing; such Biblical figures as Abraham and Jacob had multiple wives, and that's still universally understood to be completely appropriate and normal in today's marriages.
So in conclusion, I hope people who want to redefine marriage will look carefully at these examples and admit that our understanding of marriage has never changed one iota; its definition is universal, timeless, and constant.
(I probably should've checked the above stuff about Greece with, say, a Classicist before quoting it, but I'm not really all that concerned about precise accuracy in this particular entry.)