what causestom to realize tyat his wife has been having an affair with gatsby?

During the luncheon, Tom realizes that Gatsby and his wife are romantically involved. Gatsby stares at Daisy with undisguised passion, and Daisy recklessly remarks, within earshot of Tom, that she loves Gatsby.

In this chapter, Nick describes a scene at the home of Tom and Daisy at which Gatsby and Daisy cannot hide their passionate interest in one another. As Tom watches their interaction with one another, he becomes certain they are involved. When Daisy, out of boredom, suggests they go into New York to do something to relieve the boredom, Tom watches how they stare at one another. He becomes quite certain that they are having an affair and that they are intimately involved.