Jesse wrote a lengthy post attempting to debunk Courtney Martin's piece in TAP today and Dylan Matthews wrote a long post on TAPPED debunking Jesse's post. It seems that when you talk about the legitimacy of marriage, you strike a nerve.
Marriage is entrenched in a lot of stereotypes and bad connotations across the board. Jesse's argument that we're past the bad things in marriage doesn't really do justice to the fact that for a really long time, marriage has been reinforcing patriarchal culture. This of course is less of a problem for men in heterosexual partnerships than it is for women. Couples that co-habitate tend to see a sharp spike in the women's share of domestic duty and a lag in men's share. This affects couples that live together and aren't married as well, but there are a lot of things about making it legal that reinforce the old stereotypes about the happy homemaker housewife in very subtle ways. The bottom line is that the interpersonal sexual politics of relationships are really tricky to navigate.
Coincidentally, I'm actually reading Stephanie Coontz's Marriage, A History, which debunks a lot of the notions about what marriage "should" be. Basically she examines every assumption we have about marriage today and finds one (or many) societies that just never adhered to that notion of marriage. Marriages have a lot of polygamy, racism, sexism, diplomatic, and social structure histories that show the definition of marriage has changed vastly over time. Furthermore, marriage tends to reflect the values we hold in society.
Every time I get into an argument at a bar about how women shouldn't have to take their husband's names when they get married, I realize that the tradition of marriage is a highly personal thing, even if the implications of the definition of marriage have a much broader impact. For whatever reason, marriage in America today tends to be, on the whole, highly sexist and homophobic still, even if a lot of the taboos about inter-racial marriages have been shrugged off. The reason for this is because American society is still largely sexist and homophobic. Just because it's 2008 doesn't mean those things have stopped existing in society.
In short, Martin is right to be wary of an institution with such historically awful implications. But Jesse is also right that we can remake marriage into something that's more egalitarian if we want to. The problem is that we have to make society as a whole more egalitarian. Both of those things are pretty hard to accomplish.