Over a year ago, I wrote a post defending multiple meanings of marriage from a post-modern perspective. I noted that a multiple-meaning paradigm allows us to recognize as "marriage" types of ceremonies that we ourselves wouldn't or can't participate in. That might sound dangerous, but in reality it's essential: Jews should be able to recognize a "union of two people under Christ" as a marriage, and Christians should be able to recognize a "union of two people under a Ketubah" as a marriage, even though both experiences are beyond each groups respective personal horizons. Marriage would lose a lot of its transcendental character if we couldn't make such leaps, or if we blithely argued that recognizing Christian marriages somehow "degraded" Jewish ones, or vice versa.
Anyway, Eugene Volokh today has a great post analyzing and refuting the idea that their can be only one true "meaning" or "purpose" of a given act or institution, be it sex or marriage. As usual, he says it better than I ever could, so go forth and read.