Infidelity: what an interesting choice of words! According to Wiktionary, the primary definition of infidelity is "unfaithfulness in marriage or other moral obligation" with a synonym of "betrayal."
Before throwing quotes out, let's label the infidelity first. Tom can be accused of infidelity due to the affair with Myrtle. Myrtle can be accused of infidelity due to the affair with Tom. Daisy can be accused of infidelity due to her relationship with Gatsby. Due to the second half of the primary definition, perhaps Gatsby can be accused of infidelity due to the fact that he's a bootlegger in love with Daisy (i.e. he nixed his moral obligation to society by breaking the law for money as well as the law of love and marriage).
Now that the main culprits have been named in regards to infidelity, it's time to enter the realm of quotations. Tom's infidelity is proven at the very beginning of the book with Jordan's simple remark, "Tom's got some woman in New York" (15). There you go: Tom is having an affair. This is further proven when Tom takes Nick to "stop by the ashheaps" just a few pages later because Tom insisted to Nick, "I want you to meet my girl" (24).
Myrtle is, perhaps, even more bold about her infidelity, flirting with Tom directly in front of her husband while wetting her lips:
She smiled slowly and, walking through her husband as if he were a ghost, shook hands with Tom, looking him flush in the eye. (26)
The "party" at Myrtle's apartment is absolutely filled with quotes mocking the institution of marriage and praising adultery.
"Neither of them can stand the person they're married to. ... Can't stand them." She looked at Myrtle and then at Tom. "What I say is, why go on living with them if they can't stand them?"
"Doesn't she like Wilson either?"
The answer to this was unexpected. It came from Myrtle, who had overheard the question, and it was violent and obscene. ... "I married him because I thought he was a gentleman," she said firmly. "I thought he knew something about breeding, but he wasn't fit to lick my shoe." (33-34)
There's a definite irony in the impassioned conversation between Tom and Myrtle about "whether Mrs. Wilson had any right to mention Daisy's name." As Myrtle aptly screams Daisy's name again and again, Tom breaks her nose with his open hand (37).
Ah, but I can talk about infidelity, Tom, and Myrtle all day long. Let's approach Daisy and Gatsby now. As characters of betrayal, Daisy and Gatsby are a bit harder to find quotes for. Their meeting at Nick's cottage is so very awkward and full of meaningless pleasantries, there's hardly true betrayal going on there quite yet.
In regards to Gatsby and his unfaithfulness to moral obligation, Gatsby lies and lies about his past and his money. There are plenty of quotes pointing to this, but my favorite is this: "And with this doubt, his whole statement fell to pieces, and I wondered if there wasn't something a little sinister about him after all" (65). And, of course, in regards to betrayal of moral obligation taking a married woman away from her husband is certainly evidence of this, so I put this to you as the best quoted example:
She never loved you, do you hear? ... She only married you because I was poor and she was tired of waiting for me. It was a terrible mistake, but in her heart she never loved any one except me! (131)
Betrayal at its best. So refreshing, isn't it?