Given their past, the suggestion that the effect of their wedding could be to maintain respect for the institution of marriage would seem absurd.
There are many examples of royalty intent upon dignifying a politically problematic relationship with the title of marriage. The list includes Nero, Edward IV and Edward VIII. It has generally been an indication of insecurity and self-absorption and tends to end badly. A monarch needs the confidence to regard the role of his mistress as good enough for anyone.
With the advent of DNA testing marriage is now obsolescent even for royals who are going to have children. For those who aren't it is an irrelevance. How many more generations of expensive divorces can the family afford? The precedent of a long-term unmarried relationship would be a valuable legacy for Charles's sons.
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