Which character's were unfaithful to their spouses or partners in The Great Gatsby?I understand that Tom cheated on Daisy with Myrtle, but did...

Yes, we find out that Daisy has been coming over to
Gatsby's house most afternoons after their meeting.  Consequently, Gatsby stopped having
parties, and he dismissed all his staff, hiring new servants who would not gossip.  In
Chapter 7, he tells Nick,

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Daisy comes over quite often--in the
afternoons.

It is clear that
Daisy and Gatsby have had a physical relationship because Daisy flaunts it in front of
Tom.  At her own house in the presence of her husband, she kisses Gatsby.  Gatsby, also,
is so sure that Daisy will leave Tom and go with him.  When Tom got wind of the affair
between Daisy and Gatsby, he did some investigation of Gatsby's past and present
business and found out that Gatsby was involved in bootlegging and some other unsavory
business dealings.  All these clues let us know that Daisy and Gatsby have been having
an affair.

Another more minor episode of cheating is Nick's
relationship with Jordan.  Although he has a girlfriend back home to whom Nick writes
faithfully, Nick pursues a romantic relationship with Jordan while he is in New York.
 Even though he does not actually have an affair with Jordan, he does express interest
in her while at the same time signing his letters "love, Nick" to his hometown
girlfriend.