Jay honestly thinks that Daisy married Tom only for money. It was her only security into the East Egg way of life. He told her that he would return, and he finally did. He knew that she had married while he was away, but again, he knew she only did it for the money. He didn't think that she cared for him at all. After all, he had his affairs all the time, and Daisy was not ignorant of his behavior. She just didn't ever confront him about it.
When Jay came back into her life, he assumed that she would drop everything and go away with him. However, when they are in New York City, Jay tells Tom that his wife never loved him. When put on the spot, Daisy honestly says,
“Oh, you want too much!” she cried to Gatsby. “I love you now—isn't that enough? I can't help what's past.” She began to sob helplessly. “I did love him once—but I loved you too.”
So she did love Tom. This is Jay's revelation in the story. He realizes that it's not going to be as easy as he thought it was. She's not going to just walk off with him into the sunset. He even faltered when Daisy introduced him to her daughter Pammy. That was an awkward moment. His perception of their love was off enough to alter his happy ending.