The recent death of Prince Henrik, consort of Denmark's Queen Margrethe II, has brought attention to an aspect of European royalty that has been addressed several times but often leaves confusion among those unfamiliar with royal protocol. For better or worse, the late Prince Henrik will definitely be known for his well-publicized annoyance with never having been made king consort. “My wife has decided that she would like to be Queen, and I’m very pleased with that,” he declared in a 2017 interview. “But as a person, she must know that if a man and a woman are married, then they are equal.”
In many ways, it is difficult to argue with the prince's logic. The wife of a king is typically (barring exceptional circumstances) made queen consort, accompanied by holding the feminine equivalent of his title, the privilege of being addressed as "Your Majesty", and sharing equal ranking alongside him. For the husbands of reigning queens, however, this has not usually been the case. An official title such as Prince Consort might be granted, but these men have rarely been elevated to the rank of king, remain as "Your Royal Highness" rather than "Your Majesty", and are expected to walk one step behind their wives and bow to them as all other secondary members of the royal family do.
Perhaps the best way to explain this is to consider the fact that historically, most monarchs have been male, and a man who holds the title of King has generally been considered to be the reigning monarch and thus in the position of power over all other members of his family. This would include a king's wife, even though she may be his equivalent by rank she nevertheless does not share his power. The argument could be made then that if a woman succeeds as a reigning Queen, and her husband were to be made King, it leaves the assumption that he holds the royal authority over his wife. The specification would have to be made, therefore, that he is King consort and not the reigning King.
Surprisingly, there have been instances in history where the husband of a reigning queen has been made king. The best examples have taken place in Portugal and Spain. Only one man has ever been a consort to a reigning queen in Spain's history - Francis of Bourbon, Duke of Cadiz. He married Queen Isabella II, at which point he became His Majesty King Francis of Spain. He did not take on any constitutional responsibilities and did not share authority with his wife, but he was nevertheless made king.
Portugal had two female monarchs in its history (both named Maria), and during the reigns of their wives both of their husbands were regarded as co-monarch alongside their queens. However, there were specifications for a male consort becoming king under Portuguese law. The queen's husband could only become king after his wife gave birth to a male heir. The first queen, Maria I, was in fact married to her uncle, Pedro, a Portuguese prince in his own right. They were already married and had a son before Maria ascended the throne, so he automatically became King Pedro and her co-monarch when she assumed the crown. The second Portuguese queen, Maria II, was already on the throne when she married her two husbands. Her first husband, Auguste de Beauharnais (a grandson of Napoleon Bonaparte's first wife, Josephine), was named prince consort upon their marriage but died before he and his wife could have children, thus never becoming king. Maria II married a second time to Prince Ferdinand of Saxe-Coburg, who was also created prince consort on marriage but became King Ferdinand upon the birth of their eldest son.
The scenario of having a king consort actually occurred in English history once before. Queen Mary I, the eldest daughter of King Henry VIII, married King Philip II of Spain. Philip was technically her king consort though he was permitted to be regarded as co-monarch provided that all of his actions as king were performed jointly with the queen. The case of William of Orange, who became King William III and reigned jointly with his wife, Mary II, was an exceptional one in that he had been invited to take the throne by Parliament and continued to reign in his own right after his wife's death. Mary II's sister, Queen Anne, married Prince George of Denmark but he did not become king consort, instead remaining a prince and given the additional title of Duke of Cumberland. The husbands of Britain's next two female monarchs also remained princes during their wives' reigns - Prince Albert, the husband of Queen Victoria, was given the title of Prince Consort, while the present royal consort, Prince Philip, holds the additional title of Duke of Edinburgh. In the cases of Prince George, Prince Albert, and Prince Philip, neither of them held any constitutional responsibilities, neither of them could supercede their wives' authority, and neither of them were eligible to inherit the throne for themselves through their wives (though Prince Philip had in fact been very distantly in line for the British throne in his own right, as a descendant of Queen Victoria).
Is it possible that the husband of a female monarch could be named king in the future? Certainly. The high number of female royal heirs means that, in a few decades, Europe's monarchical landscape will be dominated by reigning queens. Norway will one day have Queen Ingrid Alexandra, Sweden's next monarch will be Queen Victoria, followed by her daughter the future Queen Estelle, Belgium's next monarch will be Queen Elisabeth, Spain's next monarch will be Queen Leonor, and the Netherlands' next monarch will be Queen Catharina-Amalia. Who is to say that the future husbands of these upcoming queens regnant could not be kings consort? It only takes one for other monarchies to base precedent upon.