In one interview, Puzo listed what he felt were the requirements to write a bestselling novel. One of these was to have a female protagonist.
The main female character was Kay. In some way, we (the readers) are like her: We enter the strange world of a mafia family.
And remember, she's the protagonist of the very last scene.
So it's possible that the Lucy Mancini plotline was added by Puzo to to create female interest, along with prurient interest.
Looks more like he added Lucy to create some male interest.
