IIRC, the infodump cited upthread covers this as well. To whit, the spouse must be a specified number of degrees removed from the line of succession to any title to qualify as a 'commoner'. How many, we weren't told, but Cathy definitely doesn't count: absent heirs of her brother, the Earldom could revert to her oldest child or heirs thereof.
munroburton wrote:Weird Harold wrote:I suspect that the wording, rather than the spirit would be the deciding factor. IOW, Cathy Montaigne could marry the Heir now that she's abdicated her title. (or some other Noble who abdicates a title...)
Seems like a remarkably obvious way to get around the matter. But that might be permissible, as it prevents the Crown from accruing aristocratic property.