For the Netherlands, and probably for more monarchies, it would clash with the Constitution where it is worded that the Bearer of the Crown is King, and no one else. When the Netherlands got a female monarch (for 123 consecutive years...) a special law was needed: "When the kingship is executed by a Queen, all references to The King in all acts, decrees, treaties, must be read as "The Queen" including all necessary grammatical and linguïstic changes as a consequence of this".
I think it is easier in all monarchies, for sure now Sweden, Norway, Belgium and Spain will see male consorts too, that this title is made gender neutral and not the same as he one used by the bearer of the crown.
In Sweden, the consort of the king is not only known as "queen", but actually as "Queen of Sweden" (Sveriges Drottning) , which is presumably the same title Victoria will have as a reigning queen. That is indeed a mess, which was probably overlooked before 1980 because there could not be female sovereigns in Sweden back then. At least in Denmark, they can make the distinction between Dronning af Danmark for a queen consort (as Queen Ingrid was called) and Dronning til Danmark for a reigning queen (although Queen Margrethe actually calls herself Danmarks Dronning instead).
Belgium, Spain and the Netherlands seem to have taken the alternative route of making the king's consort a queen by courtesy, but not "queen of [country's name]" or "queen of [the country's people]". Queen Máxima is still legally a Princess of the Netherlands and a Princess of Orange-Nassau despite being referred to as queen, as BTW Queen Paola and Queen Mathilde are also still legally Princesses of Belgium.
Note , for example, how King Albert II is referred to in Prince Philippe's marriage certificate below as "Sa Majesté le Roi Albert II etc., Roi des Belges, Prince de Belgique", whereas Queen Paola is cited as "Sa Majesté la Reine Paola etc., Princesse de Belgique", but not "Reine des Belges".
PS: Still on Duc's point, the way the legislators in Sweden seem to have circumvented the confusion between a reigning queen and a queen consort in the constitutional acts is by using language such as "the King or Queen who occupies the throne", or the "the King or Queen who is the Head of State", or simply "the Head of State" without actually mentioning the word "King" or "Queen".« Acte de mariage
N° 708 L'an mil neuf cent nonante-neuf, le quatre décembre, à dix heures, devant Nous, Messire François-Xavier Chevalier de Donnea, Ministre d'Etat, Membre de la Chambre des Représentants, Bourgmestre de la Ville de Bruxelles, Officier de l'Etat Civil, Grand Officier de l'Ordre de Léopold, assisté de Marceline Van Baerlem, Echevine de la Ville de Bruxelles, comparaissent à l'Hôtel de Ville :
Son Altesse Royale le Prince Philippe Léopold Louis Marie, Duc de Brabant, Prince de Belgique, Sénateur, Grand Cordon de l'Ordre de Léopold, titulaire de diverses distinctions honorifiques étrangères, né à Bruxelles, deuxième district, le quinze avril mil neuf cent soixante, domicilié à Bruxelles, rue Brederode 16, fils majeur de Sa Majesté le Roi Albert II Félix Humbert Théodore Christian Eugène Marie, Roi des Belges, Prince de Belgique, Grand Maître de l'Ordre de Léopold, titulaire de diverses distinctions honorifiques étrangères, et de son épouse Sa Majesté la Reine Paola Margherita Maria-Antonia Consiglia des Princes Ruffo di Calabria, Princesse de Belgique, Grand Cordon de l'Ordre de Léopold, domiciliés à Bruxelles, d'une part;
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