Kenji Yoshino, law professor at NYU, and author of Covering: The Hidden Assault on Our Civil Rights gave a Chautauqua Lecture at EKU. He discussed this idea of "covering" as a kind of discriminatory demand made against many people (in the workplace, the culture, etc.), not to be "too X," where the X is some feature of the person's own identity (such as, in Yoshino's case, his homosexuality). He did a nice job of taking the audience from his X to seeing how there are all sorts of features (from race, to one's identity as a mother, to one's love of, say, poetry) which can elicit unreasonable covering demands--sensibly stressing as well that not all discrimination is equal, and also speculating that we are nearing the point where challenges to fine-grained forms of discrimination will have to be "cultural" rather than legal (because the law is a meat cleaver and not a scalpel).
Enough introduction. Yoshino spoke at EKU in part as a balancing-act which brought Robert George to campus a couple weeks earlier. George is one of the more well-known opponents and voices of the anti-same-sex marriage camp, rooting his position in natural law. (I was unable to attend his talk, but apparently he did not address same-sex marriage, but talked more generally about natural law--apparently to the disappointment of many members of the audience.) However, the question of same-sex marriage, and of Yoshino's own critiques of George, did come up at the Q&A at Yoshino's talk. In particular, a professor at EKU who has been outspoken in his own opposition to same-sex marriage as well as to EKU's newly adopted policy which provides domestic partner benefits, challenged Yoshino to provide a definition of marriage.
Yoshino offered his sense of what marriage has come to mean in the culture, while criticizing (what he takes to be) George's idea that there is an essence of marriage, an ontology, which simply makes same-sex marriages not real marriages by definition. [Update: Rob suggests below that Yoshino mischaracterizes George's view and offers some corrective comments and links. Thanks, Rob!] I've always found this move fishy [Update: and whether it's the move George makes or not, something like an "argument from the natural definition" seems to crop up in discussions of this issue], and think now I can offer a conceptual distinction that makes clear why. Some concepts are "natural concepts"--they pick out natural kinds, like "dog" refers to dogs. We could choose to use the word "dog" to refer to things like cats, but in this case I think the use would then deviate from the natural concept. This is very quick, but the idea is that there is something like an essence to natural concepts, something like an essence of doginess, of doghood.
But some concepts are cultural. That is, they are the products of human culture, history, and institutions. Such concepts have practical purposes, but not essences in the way natural concepts do. I'm inclined to think that person is such a concept, and we can trace the history of the concept of a person to see ways in which it has expanded and contracted in various times and places. (E.g. not all humans have counted as persons, and not all persons have been humans.) Cultural concepts do not have an essence so much as a use and a history. Because of this cultural concepts are inherently malleable. It seems to me that the concept of marriage is clearly a cultural rather than a natural concept, because it plays a cultural and institutional role in human society. Opening marriage up to same-sex couples thus cannot conflict with the essence of marriage because there is no essence. It may conflict with the historical scope of marriage, but as with person, history and tradition is not an infallible guide to what future use we can make of cultural concepts, or whether an institution should be opened up to other groups with parallel (or identical) claims for inclusion to those who already have access to the institution. And it's clear that those who support same-sex marriage are not simply using the word marriage in a sense that is completely disconnected from its history. In light of this, resistance to same-sex marriage turns out to look like an insistence that the way things are and have been is the way they ought to be eternally. But if I'm right, that's a completely wrong-headed and insupportable approach because it attempts to construe marriage as the sort of concept it isn't.