To put it baldly, she wasn't a virgin, and more problematically, a lot of people knew it. At the time, it was still a legal requirement that the chosen bride of the Heir Apparent had to submit to a gynecological exam, often said to be for the purpose of proving virginity, but more probably, to determine that there was no problem indicating difficulty becoming pregnant. We don't know if Diana actually passed the "innocence test", but she had led a discreet enough life up until her engagement that a physician was willing to say that she had. They could not have pulled that off for Camilla, even with the best will in the world. She was known to have been intimate with more than one past boyfriend, including Andrew Parker-Bowles, whom she would later marry. None of this was out of the ordinary for the 1970's, and made finding a bride who could pass as a virgin difficult, which is why Mountbatten, who was a roaring sexual hypocrite, convinced Charles, at the age of 32, to propose to a 19 year old.
