My understanding of “common law marriage” is the institution whereby a man and a woman (let’s leave all discussion of “same-sex marriage” out of this thread) move in together, and they present themselves to others as husband and wife, and by doing this they legally become husband and wife, without needing any formal ceremony or marriage license. Common law marriage, as defined, used to be widely recognised in the United States, but nowadays most states have abolished it, and only a handful retain it.
So, let’s consider a man and a woman who live in a state which still recognises common law marriage, and they do this. They’ll legally be recognised as married under the laws of the state in which they live. But would the Church recognise that marriage is valid? Well, obviously, if either the man or the woman are Catholic, the marriage is invalid for lack of canonical form. But what if neither are Catholic–would the Church recognise the marriage as valid in that case?
Suppose neither the man and the woman are Catholic, and then later one of them wants to convert to Catholicism. Would the Church accept their legally recognised pre-conversion common law marriage as valid? Or would it insist on convalidation or radical sanation?
Suppose either or both the man and the woman are Catholic, and they enter into a legally recognised common law marriage, as described. Obviously, their marriage is invalid due to lack of canonical form. (And while the requirement for canonical form can be dispensed, my reading of canon 1127 is that doing so isn’t allowed for common law marriages, since they don’t meet the requirement for a “public celebration” which it imposes.) But could such a marriage be radically sanated? Or is convalidation required?
Finally, some have suggested that the requirement for canonical form be abolished; for example, canon lawyer Edward Peters so argues. I think, whether or not he is right, there can be no doubt that it is a requirement of church-made law not of divine law or natural law, and so the Pope has the power to abolish it if he thought it was prudent to do so. But, if that happened, would it not follow that common law marriages, entered into in those states where they are legally recognised, would be valid even for Catholics?