> Why should married people have any advantage in law over single
> people, or cohabiting couples?
I see it in simple efficiency terms: until you're an adult, your parents are relatively legally responsible for you. Once you become an adult, you're, uh, "free" -- a little too free for most people, actually. You're not a corporation that survives your death. So marriage is a convenient place to draw the (legal) line between "singley" and "jointly" -- and establish Someone Else who has your (legal) back ... since the tradition of marriage has been around forever, why not hijack it and add a bunch of other rights and responsibilities to it?
That is: if you didn't attach those legal advantages to marriage, you'd have to come up with something else to replace it. And since most people seem comfortable with the idea of having someone who is "in your family" be that person, you'd just wind up with the alternative being a new check box to the legally-stripped version of marriage:
[ ] Check here to make this person ALSO your joint-legal-partner-whatever-you-call-it
It was pointed out earlier that you can go ahead and arrange for contractual mechanisms to accomplish (most of?) the same legal things, but it's one-stop shopping if you attach it to marriage. I've begun thinking that they should even stop calling it "gay marriage" -- who says that a pair of people who are the same sex have to be gay in order to get a gay marriage? There's plenty of opposite-sex folks who have been getting married without being not-gay; why start testing for that now?
I'm not even sure that in this day-and-age of the so-called "confidential" marriages (not requiring blood tests, etc.) that it wouldn't have just been easier for "Chris and Pat" to have gotten married without a fuss. :-)
/jordan