While watching the Diamond Jubilee of Queen Elizabeth II events going on recently, I noticed that the Queen's husband is referred to as 'Prince'-- not 'King'. Upon researching further, I found that Queen Victoria and Mary III's husbands were both referred to as Prince, and not King. Is there a reason that a male marrying a Queen would not receive the title 'King', while a woman marrying a King would be elevated to the title of 'Queen'?
While Queen may refer to both Queen regnant (sovereign) or Queen consort, the King has always been the sovereign. There are historical reasons for this hierarchy --in a long line of English monarchs you will find more Kings than you would find Queens. In fact, if you do not recognize Matilda's and Lady Jane's claim to the throne of England then Queen Mary I of England becomes the first female sovereign of England --and that happened in 1553, when the institution was already about 800 years old (recognzing kings before 1066).
This [long absence of a female sovereign] obviously led to the general belief that the King was the highest authority in England (hence a Kingdom and not a Queendom). And since historically Kings had declared their wives as Queens, the Queen had come to signify a lower rank in the hierarchy. Thus, Queens never bestowed the generous title of King on their husbands.
There are two exceptions of which I am aware:
There are two types of queens. A Queen Consort is the wife of a King. A Queen Regnant is a ruler in her own right, a "female king" of you will.
The husband of a Queen Consort is just the King, per the above. But the husband of a Queen Regnant is a Prince Consort. "Prince" is one level below King, and the Consort's title is held at a level below the Queen's.
Queen Elizabeth II is a Queen Regnant, having inherited the title from her father King George VI, and that's why her husband Philip is a Prince Consort.
There are instances when the King of country A marries the Queen Regnant of country B. (E.g King Philip of Spain and Queen Mary of England.) Then the husband is generally referred to as "King," because that's what he is in his own country (A). But he's really a "Prince Consort" in country B.
There is also something called the Crown Matrimonial, whereby a Queen Regnant will give "equal" (kingly) powers to her husband through her marriage (matrimony). Sweden's Queen Ulrike Eleonora did this in the 18th century. But that's rare, and is a form of de facto "abdication."
There is a very good reason not to refer to the Queen's spouse as "King" - because they aren't the King. King is a job title in the government (like President or Prime Minister, or Mayor, or Senator), not a role in a marriage.
The monarch/sovreign of a country acquires the title "King" based on their role in the governance relationship.
The spouse of the sovreign does not share the role of monarch any more than the First Lady aquires the title President, or the wife of a policeman gets a badge.
Interesting, if less than completely relevant note; if the Kate Middleton, Duchess of Cambridge gives birth to a female child, that would be the first woman in history who would have full and unchallenged right to the title of Queen of England. (Prior to this, a female child would be supplanted by a subsequent male heir). In that case, her husband would also be "Prince".
One reason, certainly in the UK, is that a woman has historically, and still, taken her title/status from her husband not the other way round. So, if Miss Jane Smith marries the Duke of Basingstoke, she becomes the Duchess of Basingstoke. But when Countess Mountbatten succeeded her father in default of male heirs, her husband did not become an Earl.
Medieval practice was different, and if men married the last female heir to a title, he assumed her father's title sui uxoris. I am not sure when this practice died out. Whilst not related to the UK, Crown Princess Victoria of Sweden's husband is merely "Prince" not Crown Prince.
I suspect as more females inherit under gender blind primogeniture, this may well change.