What code did Mayella break?

In To Kill a Mockingbird, Atticus' says "... she has merely broken a rigid and time honored code of our society, a code so severe that whoever breaks it is hounded from our midst as unfit to live with."

Atticus is referring to the "code of society" that she violated: Whites and blacks rarely mixed socially during that period of history in the south, and physical contact between different races was taboo. Instead of telling the truth that she broke this "code," she instead claimed that Tom attacked her.