Tom Buchanan applies a double standard. He thinks it is perfectly acceptable for him to have affairs, but he is incensed that his wife would do the same. Even more important to him is his notion of class superiority. He likes to pretend that class status confers moral superiority as well.
When Gatsby insists that Daisy is leaving him, Tom sneers at him. Because Tom comes from money, he disdains self-made men. Moreover, he claims to be outraged at the idea that Gatsby has made his money illegally, through selling alcohol.
Tom insists that Daisy will never leave him for "a common swindler." He further tells the others (Daisy, Jordan, and Nick) that Gatsby and Meyer Wolfsheim had been using drugstores as a front for selling grain alcohol. The illegal activities are not actually such a concern, however. Gatsby reveals that one of Tom's friends had invested in their business, and Tom is upset because the friend, Walter Chase, had gone to jail over his role in the illegal activities.