She did love Tom once, and she refuses to say otherwise. She refuses to take part in Gatsby's illusion. He idealizes their past relationship and his illusion is dependent upon Daisy loving him as much as he loved her.

Who does Daisy really love?

Eventually, Gatsby won Daisy's heart, and they made love before Gatsby left to fight in the war. Daisy promised to wait for Gatsby, but in 1919 she chose instead to marry Tom Buchanan, a young man from a solid, aristocratic family who could promise her a wealthy lifestyle and who had the support of her parents.

How does Daisy feel about Tom?

"I never loved him," she said, with perceptible reluctance. ... However, Gatsby forces them to confront their feelings in the Plaza Hotel when he demands Daisy say she never loved Tom. Although she gets the words out, she immediately rescinds them—"I did love [Tom] once but I loved you too!"—after Tom questions her.