In Chambers' vision of the marriage-free state, personal relationships would be subject to regulation, "to protect the vulnerable, to secure justice, to provide legal determinacy, to settle third-party rights and duties, and to appropriately direct state taxation and provision". But this would not involve anything like marriage or a civil union. It would be piecemeal rather than holistic: regulations about care, cohabitation, economic interdependence, parenting and so forth would exist independently of one another. This is more flexible than any bundling of these together, since it avoids assumptions about how people do or should connect different kinds of relationships. It would be based on practice rather than status, avoiding different treatment of otherwise identical relationships just because one was registered and the other not. And regulations would operate by default, with opt-outs allowed where consistent with justice.
In the marriage-free state "marriage" would now be left to private individuals or groups to define. Given the historical power of the term, however, Chambers argues that there would need to be constraints on this, such as not allowing underage marriages. More controversially, she suggests that religions should be bound by anti-discrimination laws if they wish to proselytise to children and not just adults (in a similar fashion to private clubs in the UK). She also considers issues of "internal inequality" within religions, looking in detail at the Jewish agunah problem, where husbands can indefinitely deny wives a religious divorce.
An alternative model for a marriage-free state is having relationships governed by private contract, which Chambers considers in a chapter "The Limitations of Contract". One problem here is that default regulations would still be needed. Another is that state enforcement of relationship contracts would involve violations of liberty and privacy; the alternative of compensatory fault-based alimony would be a profound violation of equality.
Returning to Chambers' critique of marriage, she begins Against Marriage with a survey of egalitarian and feminist criticisms, then in "Marriage as a Violation of Liberty" presents a political liberal critique of marriage as "non-neutral". She considers various liberal defences of marriage: for conveying social meaning, promoting gender equality, enabling caring relationships, creating citizens, or childraising. These may meet some liberal requirements — valid public motivations and compatibility with equality and autonomy — but fail on comparison with an alternative marriage-free state which can better achieve the same goals.
Against Marriage is an academic work, but one that is broadly accessible. It stays within the realm of political philosophy and largely above the detail of legal systems or recommendations, but it addresses topics of salience in most people's lives and its implications are fairly immediate. (What concrete references there are are almost all to the United Kingdom, where Chambers is based, and the broader English-speaking world.) The style is formal but also straightforward, and should pose no problems to anyone not averse to moderately formal language and philosophical precision. And Chambers positions her ideas in relation to other thinkers and strands of thought, but doesn't assume any background in political theory — I had no trouble here myself, though I have to confess to never having read any Rawls. (Chambers is a "comprehensive" liberal, but her arguments are also directed at political liberals and in some places libertarians.)
I found Chambers persuasive: Against Marriage compelled me to rethink some of my ideas on the subject and brought much greater precision to others. And even those who disagree with much more of it will have to engage with it, as an integrated and reasonably comprehensive analysis of how the state should approach marriage.