It looks like Dale Carpenter is wrapping up a fine job he did defending gay marriage over at the Volokh Conspiracy. Thank you Dale for your Yeoman's work.
Let me respond to one thing I see coming up often by gay marriages opponents. They assert marriage by definition is one man/one woman. Calling two men married is like calling a desk a chair. If we so "undefine" marriage as two men or two women, then all the other groups can similarly undefine marriage.
Two quick responses. One, if it's just a "name" we are fighting about, that seems to lend itself to a fair compromise. If two men or two women aren't "marriage," then fine, let's not call it marriage; let's call it something else. Let's call it "garriage," or "domestic partnerships," or "civil unions"; just make sure that such gay unions receive every single right -- federal, state and local -- that opposite sex marriages do.
Second, many of the things that exist "down the slope" by definition, clearly are "marriages" (or at least no one before previously argued they weren't "marriages") but we, for good reason, have decided not to permit them.
For instance, one commenter at Volokh wrote:
Once we remove the procreate roots of marriage from the definition of marriage, marriage becomes anything any individual within society wants it to be....A mother can say that since she loves her son, she should be allowed to marry her son.
But a mother and son can already procreate! And indeed, a mother/son marriage in no way violates the one man/one woman exclusive model for which these people argue! A mother and son union, if legal, would be, by definition "marriage," but we do not, for good reason, allow them. (Also keep in mind, if "marriage" can be traced to the Garden of Eden, and if, as some orthodox Christians believe, God created only two people who then propagated the entire human race, then logic tells us that the entire human race was likewise propagated by incestuous brother/sister marriages).
Similarly, if an adult man were to "marry" a nine year old girl, that too would be, by definition, a "marriage." Such marriages did exist in traditional Western history, and still do in many other cultures, and no one argued that they weren't by definition marriage. We simply have decided, again for good reason, not to recognize such marriages.
Finally, plural marriages are indeed by definition marriage. Such marriages predate Western culture and have roots that extend as far and deep, if not deeper, than one man/one woman marriages. And for all that history, no one as far as I know argued that such marriages weren't, by definition, "marriage." Again, we have decided for good reason, not to recognize such marriages.
The only two things left which arguably, by their very nature aren't "marriages" are to animals and inanimate objects. And we simply should not take seriously the argument that gay marriage will take us off the "human" slope.
As John Corvino argues,
The bestiality analogy is the most irksome of the three, since it reveals that the traditionalists are either woefully dishonest or woefully dense. To compare a homosexual encounter — even a so-called “casual” one — with humping a sheep is to ignore the distinctively human capacities that sexual relationships can (and usually do) engage. As such, it is to reduce sex to its purely physical components — precisely the reduction that traditionalists are fond of accusing us of....
And Corvino notes: "As my acquaintance Josh Goldfoot put it, 'Marry your toaster if you like, but please don't try to file a joint tax return with it.'"
Let's do a little thought experiment: Observe a relationship between 1) a man and a woman, 2) a man and a man, and 3) a man and a toaster, and then, through "natural classification," group together or make an analogy and ask, "Which two are closer to the other?" No rational individual who knows of any gay couple would group the man/man relationship with the man/toaster relationship as opposed to the man/woman relationship. It's really that simple.
Gay marriage may or may not lead us down the slope toward state acceptance of incestuous or plural marriages, but it's not because gay marriage "undefines" marriage and in doing so forces the state to recognize other "undefined" marriages.