Myrtle Wilson is set on moving her position in society up. She looks at Tom as the way to do this. Myrtle knows that her husband, George, is never going to be the kind of man she wants or provide her with the lifestyle she wants. Myrtle is convinced that Tom is going to marry her; however, Tom has no intention of ever leaving Daisy for Myrtle. He is just using Myrtle. Myrtle's sister, Catherine, tells Nick the reason Tom and Myrtle can't get married.
"You see," cried Catherine triumphantly. She lowered her voice again. "It's really his wife that's keeping them apart. She's a Catholic, and they don't believe in divorce."
Nick, of course, knows this is not true. He realizes that Tom is using this as an excuse to keep Myrtle from pushing the marriage issue. Tom is never going to leave Daisy. Daisy represents the kind of lifestyle that Tom lives. He loves being in the society and Daisy is a huge part of that. Myrtle will never be the kind of woman that would fit in his lifestyle. Tom is just using Myrtle, and doesn't realize the tragic consequences it will have on all of them.
"You see?" cried Catherine triumphantly...."It's really his wife that 's keeping them apart. She's a Catholic and they don't believe in divorce."
In Chapter Two, Nick Carraway reports the conversations in the New York apartment on the hot afternoon in which he and Tom ride the train to the city; there, Myrtle Wilson's sister tells him the purported reason why Tom has not divorced Daisy for Myrtle. However, Nick knows that Daisy is no Catholic.
Obviously, Tom has conjured up this tale as a means of forestalling Myrtle from pressuring him to obtain a divorce. For, a man of his social and economic stature would have no intention of marrying a woman from the Valley of Ashes who lives over a garage. She is simply an amusement for him until the next comes along as Tom Buchanan is a philanderer.
This incident, along with the previous description of the Valley of Ashes, establishes the parallels between the physical wasteland of industrial by-products and the spiritual wasteland of the amoral lives of the Jazz Age.
It's not clear from the text that Tom is spreading the story that Daisy is Catholic, but it would make sense for him to do so. If Daisy were Catholic it would presumably make it harder for Tom to get a divorce, and this would give him an excuse for not marrying Myrtle. On the other hand, it might be that Myrtle knows full well that Tom will never marry her and has spread the story about Daisy's Catholicism to appease her sister and friends. She would have every incentive to do so to keep them from pressuring her about marrying Tom. After all, Catherine is the one who says Daisy is Catholic, and, although she implicitly cites Tom as the source for that information, Tom never says a word yea or nay about Daisy's religion in the novel.
If we remember how Tom and Myrtle met on the train, where Myrtle saw not Tom, but his fine clothes and shoes, and if we remember the way Myrtle is constantly getting Tom to buy her things, no matter how silly and trivial, we might conclude that Myrtle understands the nature of her and Tom's relationship. Further, Tom shows himself in this scene to be brutal enough when he makes fun of Myrtle's lower-class status by suggesting that she give McKee an introduction for photography work on Long Island. He likely wouldn't care whether there were a cover story to "normalize" the affair.