Why did Prince Charles marry Diana instead of Camilla?

Diana was chosen by Charles and Camilla out of the English aristocrats because she was young, a virgin (a requirement at the time, although this probably could have been waived since Prince Andrew's wife was not a virgin when they married), and was thought to be a "mouse" who would be quiet and let Charles have his affairs. Charles wanted to have children. He wanted HIS line to succeed instead of his brothers' or sister's line. At the time of his marriage, his sister Anne already had a son and a daughter, so his heirs weren't really needed to keep the monarchy going.