Morganatic marriage, aka “left hand marriage,” was part of ancient Frankish law but went against Church and international law. It was a form of marriage between two people of unequal status, signified by the giving of a “morning gift” after the consummation of the “marriage.” No dowry or brideprice was given, and the families of the people involved did not get into negotiating a marriage contract. A morganatic marriage could be ended unilaterally at any time. (And they often were, if a lord got a full marriage prospect that paid off better.) But it was still one step above being a mistress, and there was only supposed to be one morganatic marriage at a time. But since they were enacted in private and not in church, morganatic marriages could potentially make morganatic bigamy pretty easy.
This sounds really, really familiar. Some of the ugliest “breakups” I know of have been years– decade plus– modern day versions of this, although all the cases I can think of, the woman left.
More details, including what they turned into later, at the link.