Why did charles and diana marry if they were not in love but charles was in love with camilla?

They wanted him to marry a woman with royal blood in order to continue the royal blood line. Diana had strong royal blood. Camilla doesn't. The queen arranged a marriage between the two. Neither of them knew each other. It was purely a marriage of convenience. It was doomed from the start. Also Camilla was married at the time, and in her 40's. Diana was young, early twenties and the perfect child bearing age.