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societal ideal of a long-term, co-housed relationship with a man, often involving children. And as you say, any such relationship, whether formalized or not, will involve exploitation to a greater or lesser degree depending on the individuals involved. So in such instances, especially when there are children, at least marriage provides some protection, however minor.
For example, when my parents divorced, our single-mom family was quite poor, but our circumstances were not as desparate as those of the never-married, single-mom families in our neighborhood. If nothing else, a man who wants a divorce has to show up for court proceedings that will also determine his child support responsibility. Not so for the dad of my best friend growing up, who skipped town the moment her mom told him she was pregnant, and could never be found to secure a child support judgement against.
I think the answer is not to do away with marriage, but to work harder on the second half of that stalled revolution, as Hochschild argued in The Second Shift - to change cultural attitudes so that men are equal participants in the life of the home, so that women and children are not expected to take the man's name, etc., etc. To make all of those things you list as "personal variations" and "exceptions to the rule", and work harder to make them the norm.
And you know what is currently a primary mover in that direction in our culture? Same-sex marriage. When both partners are men, or both women, the gendered expectations no longer apply, and new paradigms are created. Yes, some same-sex marriages ape traditional marriage, with one partner assuming primary responsibility for the home, freeing the other to achieve greater earning power. But unlike heterosexual marriage, where the majority of women have primary responsibility for the home regardless of their work outside it, relationships of this nature are not the majority of same-sex marriages. Simply by virtue of both partners having gender privilege, or both partners lacking it, same-sex relationships have greater income equality on average, and similarly have greater equality in dividing the day-to-day, mundane stuff of life, the stuff that in a heterosexual relationship, tends to fall mostly on the woman.
And you know what? The conservative freakshows KNOW it. That's why they harp on about same-sex marriage destroying the institution of marriage. What they mean is, same-sex marriage threatens their cherished concept of marriage as "I get someone to bear my children and cater to my needs while I go out and chase success".
And especially when they talk about kids...oh, boy. They cite studies done on single-parent families, and go on and on about how the results mean that children need both a mother and a father. No, the results mean that children do better in homes where two loving adults are caring for them, looking out for them, teaching them. So why do the conservatives insist on using that specific, inaccurate talking point, that "children need a mother and a father"? Because they are super-invested in traditional gender roles, and they're scared shitless that the gendered expectations currently built into marriage will fall quickly by the wayside if kids are raised in homes that don't model these roles as The Way It Works.
And once again...I think they're RIGHT. 
