Tom tells Myrtle that Daisy is Catholic, and for that reason he can't divorce her. This shows Tom's contemptuous treatment of Myrtle: not only does he lie to her, he tells an outrageous lie: it is impossible that class-conscious Tom, who is so worried about the growth of non-Nordic populations in the world, would ever marry a Catholic, at that time a faith associated with non-Nordic southern and eastern Europeans.
Catherine, Myrtle's sister, tells Nick too that when Tom and Myrtle finally marry, they will move out west until things "blow over."
Later, Tom and Myrtle fight over whether Myrtle can talk about Daisy. Myrtle taunts Tom by saying the word Daisy over and over again. Tom gets angry and hits Myrtle with an open hand. Nick says this breaks her nose: whether or not it literally breaks her cartilage, it causes a nosebleed.
We see in this chapter what a miserable person Tom is to be around: whenever he arrives, the fun goes out of an event. We also understand how desperate Myrtle must be for an alternative to her life in the Valley of the Ashes to stay with a man who treats her so abusively.