It is understanding why this is such a confusing subject. In history, there have been kings rule with their wives, despite the queen being queen in her own right. There are two most notable queen who had kings: Queen Mary I and Queen Mary II, who ruled with King Philip II of Spain and King William III of England respectively.
There are reasons for their title “king”. The first is that they were already kings in their own country. Philip was the King of Spain and other countries after the death of Charles V, the Holy Roman Emperor. William was already King of Orange. They had to be known as King of England and Ireland (and Scotland in William’s case, where he was William II).
The monarchs ruled together over England, Ireland and Scotland (in William and Mary II’s case). Queen Mary I had her parliament draw up an Act, which stated that Philip would be styled the King of England and official documents would need to include both names. However, she also imposed strict conditions. England would not need to offer support to Philip’s military operations and Mary had to approve any foreign appointments into the English office.
The conditions were not the case when it came to William and Mary. Despite Mary being queen in her own right because she was the elder daughter of James II of England (James VII of Scotland), the English people wanted William to become king. In fact, William came to England to depose James II because he was an unfavourable monarch due to his catholic views.