While local custom varied, one really only had to seize on the theory that marriage was made by consent alone, which it has been shown was widely known, if not always accepted. If a couple simply moved in together, which certainly happened, there would be no certainty as to whether they were married. If one partner wanted out, all they had to claim was that they had not exchanged present consent** and that they, in fact, did not consent to be married to this person in the present, making the relationship one of concubinage instead. The court would then be stuck with a clandestine marriage case with little proof on either side and was just as likely to separate them as declare them married. Or, if they both consented, all they really had to do was separate.
Then there are the cases where everyone knew the people living together were not technically married. The most prevalent example of this would be priests’ wives and concubines. For all intents and purposes, these local priests, their partners, and most or all of their parishioners considered them married. In the 12th century and afterwards, however, the “wife” was legally and theologically a concubine. In the later Middle Ages, as the law began to crack down on concubinage, more and more cases involving ordinary laypeople begin to show up in court records, showing just how common such relationships were.** Records show, for example, that about 10% of the population of Montaillou lived in concubinage in the early 14th century. Concubinage, however, was not an easily defined category and the ways people saw their own relationships did not necessarily match up with how the law saw them.
*As opposed to future consent
**Brundage points out that most of these were short-term relationships, possibly because they had had less time to settle in and be accepted as part of the community, but a few couples had been together for a long time and had several children together. Not all of them were technically couples either. Brundage cites the examples of a man who lived with two widows and Margery Port, who was the concubine of two men at the same time.
Brundage, James A. Law, Sex, and Christian Society in Medieval Europe. Chicago: University of Chicago Press, 1987.
Dübeck, Inger. "Women, Weddings and Concubines in Medieval Danish Law." Scandinavian Journal of History 17.4 (1992): 315-322.
Rawson, Beryl. “Roman Concubinage and other De Facto Marriages.” Transactions of the American Philological Association 104 (1974): 279-305.