As the novel was written through the depression era, blacks were still highly discriminated. They were not allowed to associate with whites in public. This is shown in the courthouse with the physical separation of races and in the obvious separation of black and white areas in the town of Maycomb.
But more than anything, intermarriage was basically unheard of and deeply frowned upon.
The main example that this is shown in is when Tom Robinson, an innocent black man is accused of raping a white woman. Another