Short Answer
Given this background, should I push for him to seek a formal divorce,
or is it unnecessary after so much time has passed and both have new
relationships and plans?
A formal divorce (or a formal court declaration that there was never a common law marriage, in a divorce or annulment case, where the existence of the marriage is a core legal issue) is almost certainly necessary.
The passage of time, new relationships, and new plans are legally irrelevant.
The consequences of not doing so could be disastrous, even if the issue wasn't raised legally for decades.
Long Answer
There Is No Such Thing As A Common Law Divorce
Once a common law marriage comes into being, it continues to be in existence until it is dissolved by divorce granted by a court. See also here. The marriage continues to remain in force, even if the couple are separated for years or even decades. I've seen common law marriages in my law practice that were recognized despite a twenty year separation. There is no such thing as a common law divorce.
A common law marriage, once it comes into existence, has the full legal force and effect of a marriage entered into with a marriage license and a legally recognized wedding that is evidenced by a marriage certificate.
Was A Common Law Marriage Formed?
The District of Columbia, where they were domiciled, recognizes common law marriage.
A common law marriage that is valid where it is formed, is valid everywhere:
If a common-law marriage is valid (legal) in the state in which it
began, all states will recognize the marriage.
Of course, a common law marriage only arises if both spouses were eligible to marry each other. A claimed common law marriage is void if either of them was married to someone else at the time, if it was incestuous, if it involved someone not old enough to marry, or if one or both of the spouses did not have the mental capacity to marry. But these circumstances seem inconsistent with the facts in the question.
Under these facts, there is a very high likelihood that a common law marriage was formed. It would be very surprising if a court found otherwise, even decades later. See, e.g., here (regarding the factors establishing common law marriage in the District of Columbia). Basically, they must refer to themselves as currently married, while cohabiting, even to each other.
Referring to each other as husband and wife, and holding themselves out to family and other people as husband and wife, while cohabiting, are by far the most important factors in establishing a common law marriage. See, e.g., Coates v. Watts, 622 A.2d 25 (1993).
The question states that there are probably numerous documents that establish their marriage in school and medical records, and numerous witnesses would could be called to establish that they referred to each other as husband and wife. Testimony of a child of the couple that the child believed that the child's parents were married would also be extremely powerful evidence in favor a common law marriage, as this would reflect how the parents referred to each other in private.
A failure to merge their finances is virtually irrelevant. Many married couples these days do not merge their finances. Likewise, the wife not taking the husband's surname is much less important now than it used to be, because it is common for married women to keep their maiden names these days.
A failure to file joint tax returns (or married filing separately tax returns) is more of an issue, but is not all that serious of an impediment to finding that there is a common law marriage. I've seen courts find that common law marriages were formed without this factor in my law practice (and there are many cases where that is the case in the case law). In my law practice, I've also seen courts refuse to conclude that there is a common law marriage, despite the fact that joint tax returns were filed.
The Consequences Of Not Divorcing
If either of the members of the common law marriage purport to remarry without obtaining a divorce, this is bigamy, which is a serious crime in many jurisdictions. And, the new "marriage" would be void from the outset for all purposes and as to everyone.
The new "spouse" can't even claim putative spouse benefits if these facts are known to the new "spouse." If their "spouse" in the new "marriage" claims any spousal rights, knowing the relevant facts, they are not legally entitled to any marital benefits arising under the law. The new "spouse" might even be force to disgorge and return any economic benefits arising from the marriage (e.g. Social Security benefits or tax credits). The new "spouse" would also have no inheritance rights from their "spouse", except as an unmarried romantic partner beneficiary under a will. See, e.g., here.
This also presents all sorts of other problems that could be raised at any time. This could include a demand for alimony and for a division of all property acquired during the entire term of the marriage including the post-separation period in a divorce case, just as it could be in the case of a long separated couple that married with a marriage represented by a marriage certificate.
It could even be raised after death, in the form of a demand for a mandatory forced share for a surviving spouse, or a demand for an intestate share of the estate if there is no will, in a probate proceeding. But a claim of a common law marriage first asserted after the other spouse is dead, when the other spouse can't testify in the case, is viewed with more skepticism than a claim of common law marriage raised while both purported common law married spouses are alive. See Coats v. Watts, above. In D.C., such claims must be established by clear and convincing evidence, rather than a mere preponderance of the evidence (i.e. that it is more likely than not that the facts are true).
In a few states, such as North Carolina, the new "spouse" could even be sued for hundreds of thousand of dollars or more for interfering with the existing marriage. If the "remarried" couple moved to one of those states, this kind of civil lawsuit could be brought in that state since the original marriage is still in existence during that time period.
Many states have repealed their adultery statutes, or adopted a stated or unstated policy of not enforcing them. But the common law marriage would expose both parties (if the both know of the prior common law marriage) to criminal liability for adultery in the states that still have this crime, even prior to purporting to "remarry".
Would A Statement That They Weren't Married In Custody Litigation Matter?
There is a remote possibility that a court ruling in a child custody or child support case that the parents weren't married could constitute a binding legal determination that the couple wasn't married under the doctrine of collateral estoppel. For the child custody determination to be binding in later lawsuits, four factors must be established (per the collateral estoppel link):
- The issues in the second suit are the same as in the first suit
- The issues in the first suit must have been litigated (on the merits)
- The issues in the first suit must have been decided (on the merits)
- The issues must have been necessary to the court's judgment
But since child support, parenting time, and parental responsibilities are all matters which are not in any way legally dependent upon whether or not the parents are married, it is much more likely that a court order in a child custody or child support case is not legally binding in future matters. This because the issue of the existence of the marriage wasn't resolved on the merits and there was no incentive to litigate that issue in that case, because it was unnecessary to resolve the child custody/child support issue.
