Why is Tess's character's purity still portrayed as unpure even considering the circumstance?
Tess was an innocent, but she was not pure. She had had an affair with Alec and she had gone home pregnant with his child. During that era a pure woman did not act in such a way. In fact once she confessed to Angel what had transpired between she and Alec, he didn't even want to marry her. Hardy wrote,
"She had been made to break an accepted social law," Hardy observes, "but no law known to the environment in which she fancied herself such an anomaly." Later, Hardy notes that Tess's shame was "a sense of condemnation under an arbitrary law of society which had no foundation in Nature." Victorian society, with its strict code of appropriate and inappropriate social behavior, was anything but natural."
He was also trying to write an allegory about the hypocrisy of religion during that time. Even though Tess is in all intense and purposes innocent of the first encounter with Alec, she does go to his bed again and again. Then later even though she has married Angel, she returns to Alec bed when she thinks Angel had abandoned her.