The daughter-in-law becomes the object of her lustful father-in-law's fantasies. Since he is absolute, it is a hierarchical relationship that cannot be resisted. The sexual act that takes place on top of that is dominated by a sense of immorality and elation, and she is overcome with emotions she has never felt before. It's not like it's okay with just anyone, though. I can't forget the cunnilingus my father-in-law gave me... If it feels better than sex with my husband, I guess I'll just accept it...