First, we need clarity of what actually exists. The statistics include the population down to sixteen years of age. Those are "adults"? Really? And what counts as "single"? Someone who is shacking up is not really single, are they, especially if they have children and financial accounts with their partner?
I'm sure most the change that has created this snapshot has to do with things like 1) people getting married for the the first time at older ages, 2) divorce, and 3) people living longer divorced or widowed.
Some of it, though, no matter how small, has to come from men who've gone on a "marriage strike", and I expect that to have an increasing effect on the statistics, even if in the end, many of the men end up with their strike just being a delay and not a lifelong standoff. The men who've joined the marriage strike are a diverse group, ranging from hedonists to devout conservative Christians who have come to the conclusion that legally-sanctioned marriage is something to be avoided.
Hedonists are increasingly realizing that they can get more sex, and with a variety of women to boot, if they do not marry, often without any obligation or jumping through any hoops. Hedonists (or anyone who has no moral/spiritual concerns about unmarried sex) can now get literally everything they want without ever marrying, without suffering (and often benefiting) professionally and socially. Others, such as the religiously devout, would rather be chaste and go through life "alone" than take on what they see as the risks, harms, restrictions, and burdens of marriage. Marriage strikers range from those who try to have as little interaction with women as possible (including in the workplace) to those who will do everything with women except legally marry them.
No small part of why these people have gone on a marriage strike has to do with the family laws and courts:
- unilateral no-fault divorce
- community property laws combined with the fact that most men do/will earn the majority, if not all of the income during the marriage financially punishes men for marrying
- alimony requirements (lifetime, in some places!)
- child custody and support issues
- presumed paternity and paternity fraud
- domestic violence response by law enforcement being at the point where a man can get physically assaulted by his wife and be the one to go to jail and permanently kicked out of his own home while still required to pay for it
There is probably some legislation that could ease at least some of the concerns of marriage strikers. The problem is, it is a certainty that introducing such legislation would be met with shrill shrieks of being part of some nefarious conspiratorial "war on women". That's a great fundraising tactic for certain politicians and abortion advocates.
Especially since it is increasingly becoming a legal principle that marriage is about the feelings of adults rather than what benefits children or society, I expect the statistical trends to continue, meaning we'll see the percentage of the adult population legally married at any given moment continue to shrink. After all, why bother to get or stay married to raise your own children, since marriage "isn't about children"?
This is not a matter to be ignored by Republicans. Married women are far more likely to vote Republican than women who aren't married. One reason is that many unmarried women, while claiming to be "independent", are very dependent on government programs and thus male taxpayers they don't even know, and Democrats promise more government programs. I suspect another reason is that Republican women are generally far more attractive and thus more likely to attract a husband.