Thus, an affinity emerges between Mary and her protagonist. She locates her struggle within Victor's creative work to give voice to an experience she would not otherwise be free to articulate. This is supported by some of Percy's later works, most specifically, Epipsychidion , in which, he compares Mary to the Moon. This could be understood as evidence that their relationship was in flux and definitely not the same after the death of their children. This could also be another probable answer to why Mary Shelley chose to base this story on father-daughter incest.
Incest is the worst, most despicable act of sexual behavior, opposing all morals.
It could be feasible that Mary chose an extreme to reveal her own feeling toward sexual desire. Therefore, Mary may have grown to feel that sexual behavior, if fulfilled, may lead to children, which in her case leads to their death and, consequently, her melancholy and depression. This was not worth the joy of intimate union between herself and Percy. Mary and Godwin's relationship was also quite strained at this time. Godwin was upset when Mary eloped with Percy, a married man.