The marriage between Tom and Daisy appears to be a marriage of convenience between two people of the same class. In the world of Gatsby, "old money" marries "old money." It would be unacceptable to marry someone below your own social class. Because Tom and Daisy are both from wealthy families, the match is perfect, except for the one thing that should bind a marriage, love. Tom is an abusive brute to Daisy, and professes to love her only to maintain appearances. Daisy, too, is disgusted with Tom and his affairs outside of marriage. The expectations for the rich are enormous; they are expected to maintain the social status they are born in and not fraternize outside it. Tom and Daisy are willing to be unhappy for the rest of their lives in order to "keep up the appearances" demanded by their wealth and class. In the end, Daisy and Tom leave East Egg to go to Europe so they are not linked to the scandal of Gatsby's death. Scandal is also social suicide for the rich.
Jay Gatsby never stands a chance with Daisy for he comes from a poor background and is "new" money. Fitzgerald is making a comment about sacrifices the characters in his novel make because of societal pressures and expectations. Their existence and happiness is dictated by a rigid class system that demands they act a certain way.