If you're a member of the Royal Family, you have to be sure about who you wish to marry. Very sure, in fact. Nobody of royal blood may propose to their beloved without the specific and prior permission of the reigning Monarch. This isn't merely a matter of tradition, it's a matter of law, as stipulated by the Royal Marriages Act of 1772.
During that year, George III (he of "Mad King George" fame) was so appalled by the fact that his brother had married a commoner that he insisted a law be introduced so that it never happens again, and so each royal suitor must be acceptable to the King or Queen of the time. It is believed, but can't be proven, that the Queen has once denied her sister Margaret permission to marry on the grounds that her suitor was an adulterer. For a long time, it was also illegal for any British royal to marry a practicing Catholic, dating back to the time of Henry VIII.