Dear Inky.
That must certainly be a projection of their Id's deepest unfulfilled desires due to societal and moral constructs.
In the case of abusive relationships, once again it's usually a distorted projection of a desired relationship where you have been forced hence you're not guilty. It's usual among teenagers, who often see sex as guilty and fantasize about abuse or rape where the blame can't be put on them (again, these are simple generalizations and individual cases will always differ).
When it comes to incest, Œdipus or Electra syndrome, my friend.
The desire of your opposite sex parent that occurs during youth.
Pedophilic abusive fantasies often have its root in power roles.