We don't know. Supposedly that was essential in order to marry Charles, but we've no more idea about that than we have about Harry shouting at staff "What Meghan wants, Meghan gets!".
They do indeed MissAdventure. Diana was the virgin bride, a rare thing even then in a 19 year old and vanishingly rare these days. Young people have sexual relationship before they "settle down". Diana had her experience when her marriage was falling apart. Not an unusual thing and certainly not yet another thing to use to 'prove' she was mentally ill, unreliable etc etc.
i thought everyone knew that Diana was duped and used. She was only 19 years old. She didn't know Charles wanted Camilla that is certainly the way that makes sense. She never looked like she knew what she was getting into