Whether you have a ceremony or not is not a causative predictor of a stable relationship.
The evidence in the other direction showing marital status is linked with child poverty is pretty clear. Census data shows that poverty rates are five times higher for children of single mothers (1).
Maybe the mechanism isn't clear or something, but the data is stark enough to deserve an explanation about why marital status isn't important to kids.
(1) http://educationnext.org/files/ednext_XV_2_mclanahan_fig04.j...
I would be interested to see research comparing the children of married couples to those of unmarried stable long-term relationships. My hypothesis is that the outcomes would be similar.
Understood. I still think it's fair for the burden of proof to fall on the people trying to explain away current demographic trends. Just because possible mechanisms don't line up with expectations doesn't mean the outcomes aren't actually happening.
And, at some point, there is a distinction without a difference when talking about long-term monogamous cohabitations that do or do not involve a wedding ceremony. In common law, it would be a marriage either way. Maybe the real question is why common law marriages aren't recognized more often now.
If you just want someone to stay with you, that's just a strong form of friendship, and there's no reason to want only "one" such person.
There are lots of replies about how the spouses benefit from marriage, so I'll add a new reason: children, especially "oops" babies.
About half of all pregnancies are unplanned, regardless of marital status. It's honorable to explicitly define the relationship so the current and future beings affected by a sexual relationship are well cared for. Because of economies of scale, the most efficient way to care for a family is under one roof. There are direct reasons why the poverty level of kids is directly related to the marital status of their biological parents.
The legal contract and the social contract (wedding vows in front of all your favorite people) are ways to ensure the definition of the relationship is enforced by all prudent means.
Of course, you could have sex hundreds and thousands of times during your fertile years hoping that a baby doesn't happen. And you could just get married once a birth is imminent. But over the millennia, most cultures have found that an already established marriage is the best place for a pregnancy.
Anyway, I want nothing but the best for all the single-parent and divorced families out there, but it's not empirically controversial to claim that kids born to stably married couples have the best outcomes.