It`s not like Charlie has a history of being a complete **** is it.
Charlie was 32 and Diana just turned 20 on their wedding day which took place after a short engagement. It`s pretty obvious that the heir to the throne needed to find himself a brood mare. Then taking into account the "whatever love is" answer he gave when asked if they were in love it looks like he found one.
What transpired and who`s bed he ended up in led me to believe that what went on with Diana was either planned by him, with or without the prompting of the Royal family and it`s institution.