So basically he is having an affair with Whitney and not in the sense of the word 'abusing' her because she is consenting to it. Or thats what she thinks.
She is still underage so it is still technically child abuse but in her mind they are just having an affair and nothing is wrong.
More than likely he will have groomed her into thining it is ok for them to be together and probably told her that when she is old enough he will dump Bianca and be with her.
I think the storyline could be less to do with the actual abuse but more to do with how someone can mess with your head so much and make you think something is right when its not. And how he actually groomed her into thinking this way and thinking it is ok. How a child can be trusting in the adults around them. And how a peadophile works. Making her beleive that someday they will be together properly.