It wasn't just Charles who had feelings for someone else during the marriage.
As early as 1985 Diana started an affair with Major James Hewitt whilst Charles, at the same time, had an affair with Camilla. Diana's affair lasted five years, and there are even those who believe that Hewitt is the father of Prince Harry.
By the way, when Diana married Charles she acquired the rank of third highest female in the United Kingdom Order of Precedence:
Orders of precedence in the United Kingdom - Wikipedia, the free encyclopedia