The word "King" signifies the royal who holds the power. While it is typically a man, when a woman holds power (as when her husband dies, or she is the only one to inherit) she is legally the King, though she is also a queen and is usually called the queen. I believe Queen Elizabeth was an example, but you can get a lot of them by looking up "historical female kings."
Also people should be able to call themselves as they want as per Firefly Fanatic's response.