Were ever whispers about the political implications of this marriage? The bride was significantly younger than the groom, and they did eventually get divorced, I sense that while maybe the marriage was not arranged, it was certainly a calculated step on both parties. Even though Franco had already designated Juan Carlos as successor, Alfonso could have hoped marriage to Franco's granddaughter would give him a shot at the throne (even though he was the product of an unsanctioned marriage), especially since he was the senior line of the Bourbon family. And Carmen has been known to relish in the royal spotlight.
Prince Carlos Hugo of Bourbon-Parma (err, his parents definitely) was said to have married Irene partly because of status as a reigning princess. On Irene's part, rumors were that she wanted to be a queen but she was second-born, and Carlos was a Carlist claimant to the Spanish throne. Juan Carlos was said to have given up Maria Gabriella of Savoy because Franco wanted him to marrying reigning royalty.