According to Atticus, Mayella was motivated to lie to rid herself of her guilt. What rigid code of her society did she break?

From the text:

"She was white, and she tempted a Negro. She did something that in our society is unspeakable: she kissed a black man. Not an old Uncle, but a strong young Negro man. No code mattered to her before she broke it, but it came crashing down on her afterwards."