Elaborately explain the theme of morality in Tess of the d'Urbervilles.

The subtitle of Tess is "A Pure Woman Faithfully Presented." From the title onward, Hardy is attacking Victorian sexual morality. 
Tess, a naive 15-year-old, is not well protected by her parents, who put her in harm's way when they send her to work for a supposed cousin, Alec. Alec, an older man, rapes and impregnates Tess, who is now shamed as a "fallen" woman. Hardy shows, however, that what happened to her is not her fault. He portrays her as a good young woman.
Later, at the dairy, Tess experiences living in a place where conventional morality doesn't press so strongly. She starts to believe that Angel Clare could love her despite her past. After they marry, he tells her that his past hasn't been "pure," but when she confesses what happened to her with Alec, he reacts in horror and recoils from her. He responds according to the hypocrisy of the Victorian moral code, which allows a man, but not a woman, to have sexual experiences before marriage.
While today Alec would be in prison as a pedophile rapist and Tess not blamed, Victorian audiences objected strongly to Hardy's subtitle labelling Tess a "pure" woman. To them, she was a fallen, impure woman. The public missed Hardy's point entirely. Hardy was trying to point out that Victorian sexual morality was twisted, hypocritical, and wrong in blaming women for the things men did to them. But his readers were not ready to accept that. Today, we more clearly see that it was the moral code, not Tess, that was at fault.