Tom and Daisy Buchanan

Tom Buchanan is a "hulky" wealthy man who is arrogant as well as a racist.  He is married to Daisy, who seems like a superficial, spoiled woman.  She is said to be Nick's second cousin and has a daughter with Tom.  Both Tom and Daisy live on the wealthy side of West Egg.  Tom cheats on Daisy, which does not seem to be a secret, as most people including Daisy know about this affair. 

Why would someone stay married with a person that cheats?