If you have a compelling case for why my life would be better without marriage I'd love to hear it.
I shall call this the "loving, consensual model" of a relationship:
Given that you should be indifferent between cohabitation and marriage, and marriage has non-zero costs, why would you prefer marriage?
The reason is insidious, cloaked in the positive connotations of marriage and love, but nevertheless incontrovertible.
You don't prefer to be with someone if and only if they prefer to be with you.
You prefer to be with someone.
Of course, it's illegal to directly enforce this preference. Unlawful imprisonment, and all that. So you'd go with the consensual model, but raise the costs of them preferring to be separate as much as legally possible. Like, say, requiring a contract that is costly and messy to break.
There are lots of situations where precommitting to doing something at some future time, and honoring that precommittment at that time regardless of whether I desire to do that thing at that time, leaves me better off than doing at every moment what I prefer to do at that moment.
"Marriage" as you've formulated it here -- namely, a precommitment to remain "with" someone (whatever that actually means) even during periods of my life when I don't actually desire to be "with" them at that moment -- might be one of those situations....
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.