Or how about a royal from the United Kingdom marrying a President of the US?
Why not?At least in America, he and she would not have their titles, and The President, whether it was a he or a very unlikely she, would probably not be put in the line of succession.
Why not?
"No title of nobility shall be granted by the United States: and no person holding any office of profit or trust under them, shall, without the consent of the Congress, accept of any present, emolument, office, or title, of any kind whatever, from any king, prince, or foreign state."
Doesn't mean you can't accept a title from a foreign country. Or keep what you already have.