Like just about everything else in the story, the numerous cases of infidelity highlight the fundamental vacuity of the American Dream. For one reason or another, no one seems satisfied with their lives. In keeping with the spirit of the American Dream, they're always looking for something better, hence their need to have affairs. A materialistic, hedonistic culture has spawned an attitude of contempt for all the old moral values. In this era of instant gratification, people are increasingly impatient with the notion of developing lasting, loving relationships.

What's particularly tragic for all concerned is that the dreams they aspire to realize through their affairs will never be realized. Myrtle sees Tom as her ticket out of the Valley of Ashes and into a world of opulence, wealth, and gracious living—yet she's destined to have her life cut short by Daisy's erratic driving. Daisy's brief fling with Gatsby is a way for her to escape the confines of an unhappy marriage—yet deep down, she knows that she will never leave Tom; her highly-cherished social status is just way too important. And as for Gatsby himself, his affair with Daisy is a means of recovering a romanticized past which he yearns to recreate in the present and in a future with Daisy. It is this dream by which he lives and by which he will also die.

To me, the three main instances of infidelity in this book are Daisy's infidelity to Tom, Tom's to Daisy, and Myrtle's to George.  Tom and Myrtle are having an affair with each other.  Daisy does not actually have a physical affair with anyone but is, in my opinion, unfaithful nonetheless.

Daisy is unfaithful to Tom because she has fallen in love with Gatsby.  She is willing to tell Tom that she loves Gatsby and is almost willing to tell Tom that she never really loved him.  That is infidelity that may be worse than Tom having a physical affair with Myrtle.

I'm not saying that Daisy isn't provoked, but I am saying that her infidelity might be the most hurtful.

The greatest infidelity found in the story is Tom's because he is having an affair with Myrtle and isn't trying very hard to hide it.  In fact, he practically flaunts it for Nick.  The proof comes from the fact that he is keeping a secret "love nest" in the city for her and is buying her expensive trinkets.  It is also implied, at one point, that Tom had even cheated on Daisy shortly after they were married with one of the maids for the hotel (which is discovered after the two are in a car accident in which she is injured.)

Daisy, of course, is cheating on Tom with Gatsby.  You never really see this in detail, but it is pretty well implied because of the amount of time she is spending at his house and the efforts he goes through to keep her visits on the "down low."  The story also describes how Daisy goes around the table and kisses Gatsby while Tom is on the phone in the other room (ironically with Myrtle, if I remember right.)  Unlike Tom, Gatsby tries very hard to keep his affair with Daisy quiet by firing his potentially gossiping staff and ending his weekend parties.  Daisy is a little less discrete, inviting Gatsby over for lunch and such, but in some ways her cheating can be seen as almost retaliatory against Tom.

You would have to add Myrtle cheating on her husband George.  Though she doesn't seem very attached to him, and he seems pretty clueless to her wandering for much of the affair, she is still playing house with Tom rather than honoring her wedding vows.

In some ways, you could even make a case that Daisy cheated on Gatsby.  They were involved in a relationship while he was in World War I (and afterwards, while he was stuck in Europe,) and in the end she rejected the relationship in order to get on with the life she felt she was entitled to.  Though not a marital infidelity, she still pretty much ditched her boyfriend.

Keeping with that definition, you could add Nick to that pile.  Remember, Nick was cavorting around with Jordan while Nick has made clear that he has a girlfriend back in the Midwest that he is expected to marry.

So, as you can see, pretty much everybody is unfaithful in one way or another in this story (other than maybe Jordan and George.)