The author uses a bit of dialogue between Nick and Myrtle Wilson's sister Catherine to characterize Tom Buchanan.
"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 comments to the reader:
Daisy was not a Catholic, and I was a little shocked at the elaborateness of the lie.
It is characteristic of Nick that he listens and observes but seldom contradicts people or offers information. He is a little shocked because the elaborateness of the lie suggests that Tom has invented it and improved on it over a period of time. Tom has had other affairs and has undoubtedly used the same story on his other mistresses. We know that Tom is just using Myrtle and would never consider marrying her. She belongs to a social class he despises. Tom is a "user." He is using both Myrtle and her husband. What is happening at this rather sordid party in the small apartment is that Tom is slumming. He belongs to the upper class, but he has lower-class tastes. Evidently he feels comfortable with people like Myrtle and her sister and their friends. But he is also a snob. He knows that Daisy is infinitely superior to Myrtle in every respect. That is why he evidently has told Myrtle that he doesn't want her to mention Daisy's name. Tom can cheat on his wife while still respecting her.
Myrtle is being deluded. She thinks there is a chance that she can somehow build a strong enough relationship with Tom to get him to divorce Daisy and allow her to marry into riches. She would gladly dump her poor husband if she had any chance to better herself. It seems inevitable that Tom will get tired of her, as he has with other women in the past, and will terminate the affair, probably unceremoniously and with a cash payoff. Tom's selfish, brutal character is reflected in his choice of a mistress. The fact that he ends up breaking her nose is a good indication of his contempt for her.