Tom's condescending remark refers to the fact that Daisy and Gatsby come from two very different socioeconomic backgrounds. Daisy comes from "old" money while Gatsby is a member of the "nouveau rich" whose money comes from questionable sources. Ironically, Tom thinks Gatsby incapable of class because that is a quality that money can not buy. Daisy even says that girls like her don't marry poor boys like Jay Gatsby. Even though Tom has his low class mistress, Myrtle, he still has no intention of marrying her as he reveals when he tells Myrtle that Daisy is Catholic. It is permissible for the upper crust to socialize with the nouveau rich, but it is not acceptable to bring them home.
We learn early in the story that Tom is a racist. His comment “next they’ll throw everything overboard and have intermarriage between black and white” is another example of his expression of this. The context, however, is Tom’s awareness that Daisy and Gatsby have a relationship, that Gatsby is, in his terms, “making love” to his wife. The statement of interracial marriage indicates that Tom views Gatsby as an “other,” that he looks down upon him as much as he looks down upon black people. Tom’s racism speaks to his arrogance and need to make himself appear and feel better than others. His racial slur in reference to Gatsby is an insult as well as embarrassing, for the narrator then comments that Tom saw himself “standing alone on the last barrier of civilization,” with Jordan murmuring “We’re all white here.”