Gatsby and Daisy can be seen as a relationship representing the failure of ideals, while Tom and Daisy can be seen as the success of social convention.
Gatsby and Daisy and involved in playing out a romance rooted in an idealism, hopeful but unreal; fantastic but fantasy. This view of the relationship is clear from the outset and is brought home when Gatsby demands that Daisy denounce her love for Tom say that she never loved him. This absolutism from Gatsby demonstrates the idealism of this relationship and also demonstrates its impossibility for success.
The one relationship that survives the novel is Tom and Daisy. This relationship is not based on love, exactly. Rather it was founded on love and has developed into a bond of mutual interest, of social convention, and of familiar resentments and dramas.
This relationship lasts, perhaps, because it is the one that society celebrates. Two monied people moving in elevated social circles; flaunting their wealth; outwardly enjoying their status. Certainly, this is the relationship that best conforms to conventional ideas of the time regarding the upper classes.
The relationship between Tom and Myrtle reflects the exploitative relationship of wealth to poverty and upper class to lower, while also representing the broad notions in America of the importance of money in the value system. Money equates to or negates morality in this relationship.
The relationship between Jordan and Nick is more difficult to reduce to a symbolic explanation.
"I thought you were rather an honest, straightforward person [Jordan said]. I thought it was your secret pride."
"I'm thirty," I said. "I'm five years too old to lie to myself and call it honor."
She didn't answer. Angry, and half in love with her, and tremendously sorry, I turned away.
Nick is drawn to Jordan as a member of the upper classes. She is socially adept and, importantly, also withdrawn. She is of the upper class but not truly engaged in that social scene that defines it. Nick is similarly situated though even further removed from the center of this scene. In this way, the two are both outsiders, witnesses, commentators on what goes on (between Tom and Daisy and at Gatsby's parties), and judges as well.