If we read the story carefully, we can conclude that all Daisy really wants is security in life.
This all-pervading desire may explain why she remains married to Tom Buchanan. To be sure, Tom is a pompous, difficult man, the sort of masculine creature who makes a woman uneasy with his predilection to violence. However, he is also powerful and immensely wealthy. To a woman like Daisy, Tom's obvious faults eclipse what he can provide her in terms of physical and material security.
We may conclude that Daisy's rationale is flawed, but it's clear that she understands her limitations as a woman of her time. With few skills and little ability to earn a living, Daisy has chosen to throw in her lot with Tom. She tolerates his womanizing ways because he provides what she needs most in life: absolute, uncompromising security.
Nowhere is this more evident than in the strange episode of Tom breaking the nose of Myrtle, his mistress. Myrtle's supposed crime? Impulsively speaking Daisy's name repeatedly, without Tom's express approval. Despite his affection for Myrtle, Tom has little illusions about her place in his life. To Tom, Myrtle is an entertaining distraction, nothing more. Despite his philandering ways, Tom sees Daisy as a vulnerable woman who needs his protection.
In the story, we learn that Daisy had reservations about marrying Tom the day before her wedding. But she did carry through with her resolve to become his wife. The text tells us that Tom did not fail to impress on the day of the wedding. He brought a hundred guests in four private cars and reserved the whole floor of a luxurious hotel for the reception. Additionally, he gave Daisy a "string of pearls valued at three hundred and fifty thousand dollars."
So, despite her reservations about Tom's character, Daisy remains married to him. From Tom, Daisy receives the one thing she most values in life: security.
They weren’t happy, and neither of them had touched the chicken or the ale—and yet they weren’t unhappy either. There was an unmistakable air of natural intimacy about the picture and anybody would have said that they were conspiring together.