Why does Aunt Alexandra accept that the Cunningham's may be good but are not "our kind of folks"? Should people mix only with others of the same social class?

Aunt Alexandra is stuck in the past, she thinks she's too good for the Cunninghams.... that they are beneath her. In my opinion, people from all walks of life should mix with each other..... social class is a thing of the past. There are only things I can't afford... not things I cannot do.