BBC Magazine article discusses the question of why a couple would get married after having kids, without suggesting the most obvious answer: for health insurance or legal or tax purposes. (Granted, marrying for the health insurance is perhaps not so obvious in a nation with socialized medicine.) But my point is that government- or authority-recognized marriage has only ever been an economic contract. The religious trappings and moral imperatives were added for solemnity -- to make the couple take the marriage more seriously, put them in mortal fear of their souls if they change their minds afterward, and provide some predictability in estate planning.
Anecdotally, I know an American couple who got married only for the health insurance. They've kept their marriage open, have had two kids, and are still together some dozen years later. From what I understand of their decision to get married, they flatly wouldn't have done it if one of them hadn't needed healthcare. Another acquaintance of mine has been married twice, and both times it was solely for health insurance because of a chronic condition. The first marriage was kind of predictably a disaster, but the second seems to be going well.
And in the end, that person moved to England for, as they put it, simply to stay alive. No American health insurance company, even the Cadillac insurance they got through the second spouse who worked for a Fortune 500 company, would pay for their preventive and ongoing care in a timely and reasonable manner, so they moved to a country where you just go to a clinic, get treatment, and go home.
As for marrying for legal and tax purposes, for crying out loud, what do you think the fight about gay marriage is all about? It's about hospital visitation rights, intestate inheritance issues, and mortgage interest tax deductions.
Most of the time, follow the money. Marriage qua marriage is an economic and legal question, and that's all it will ever be. Consider how easy it is to get married, compared to how complicated it is to get divorced. And have you noticed that, once you are divorced, you tend to have to identify yourself as "divorced," not "single" again? A marriage that ended years or decades ago follows you forever!