One sociologist I read a bit of recently would say that this was all part of the attempt to make the citizen "legible" to the government. This effort began back in the late Middle Ages when governments began demanding that everybody adopt surnames. Prior to this, governments really had no idea who their citizens were. By a hundred years ago, we knew what they were named, when and where they were born, what their marital status was, what their address was, and more. In 1932, workers became "legible" to the federal government through their Social Security numbers, though if you weren't a worker, you didn't have one. I still remember having to apply for a SSN when I got my first job; now, parents routinely apply for SS numbers for their newborn children; indeed, you must declare each dependent child's SSN on your tax return.
Well, it was all so much tidier than before, I guess. Before 1910 or so, if you established a conjugal household with somebody, you were married. Licenses were encouraged, but the forms of government didn't penetrate everywhere and therefore couldn't be required. By tradition, you were supposed to live together as husband and wife for seven years to be considered married, but I think that was a folk legend. In practice, if you considered yourself married, you were married; if you'd been living together for a long time, you were married whether you considered yourself married or not. But with the ending of common law and the definition of every kind of legal relation by statute, now you could only be married if you fit the State's definition of marriage and went through the proper legal process.
Which lasted barely sixty years. In 1971, Michelle Triola Marvin sued actor Lee Marvin for compensation following their breakup. Lee Marvin had been married before and been divorced. He and Michelle entered into a cohabitation arrangement which would have been considered common law marriage if the common law had still been in effect. Michelle legally changed her name to "Marvin," though they were not married. After five years, Lee dumped Michelle and married someone else with all the legal furnishings. Michelle sued for what was called at the time, "palimony." Note that there was no dispute that the two had lived together without getting married for the express purpose of not getting tangled up by the encumbrances of legal marriage. Lee thought he was free and clear, but the court disagreed, and he had to pay up as if he had been sued for divorce.
The state of marriage since has been a very mixed bag. On the one hand, you're not married until you're officially married; on the other hand, just because you're not married, that doesn't mean you don't owe somebody something if you break up. This has some interesting effects upon the debate over same-sex marriage.
1. If the common law definition had not been discontinued, gay couples could probably sue to establish their right to be considered married, and nobody could say that the court granting them that right had overstepped its bounds. The common law was always applied by courts, not by statues. Somebody who objected would have to pass a statute or amend a constitution to STOP the recognition of gay marriages. This also explains the outrage in recent years when courts have legitimized gay marriage in defiance of statutes and constitutions: part of the deal in giving up the common law was, everything would be defined by the people through their legislature.
2. When the agitation over gay rights began in the 1970s, one of the first things gay people complained of were social and legal benefits. A gay person dying in a hospital could find his or her partner of many years barred from visiting by one's estranged (but legally related) next of kin. The body of a dead person belongs to one's next of kin, which meant that gay partners could be barred from funerals of their lovers. This was an outrage, and it has been mostly eradicated. Hospitals and such changed their policies as society changed its views on gay relationships. Obtaining power of attorney and other legal rights of guardianship and survivorship is now much easier than it once was. In effect, this issue is largely moot. So with the sharing of employment benefits. Many companies now routinely allow you to declare anyone sharing your household as someone to be covered by your insurance, share in your pension, etc.
It makes one ponder the issue of unintended consequences, doesn't it? I mean, if we had never gotten so fussy over legal formulae, this would have largely resolved itself, perhaps. The definition of marriage might not be the flash point it is now.
As for heterosexual couples, the consequence of defining marriage did not mean the strengthening of marriage. As the hassles of getting married -- or getting unmarried -- increased, people began looking for dodges. Once the Marvin case showed that there is no escaping the interference of the State, we began developing no-fault divorce. The ultimate consequence here is that marriage is the easiest form of legal partnership to get into and get out of, with the consequent loss of people's willingness to invest in it.
Perhaps, if we are going to be all statutory and everything, we should reach back to the ancient Romans and have two forms of marriage. They could be married either confarreatio or diffareatio. The latter was easy to get into and out of, the former a major legal undertaking not easily shed. If you gave the people the option to aim higher, maybe they'd decide it was worth trying more.
Correction: I have been advised that Michelle Triola Marvin's award was vacated upon appeal, and looked it up. She didn't get anything after all. That said, the perception is that she did. And perception is often the molder of reality. I would argue that lots of people changed their perceptions about the difference between marriage and living together because of that case and all the chatter that followed it. Pre-nuptial agreements, largely unheard of before, began to be experimented with by those with assets to protect; the great herd of people with little on the line began to see living together as just marriage-without-the-paperwork.