Regarding romantic relationships, fidelity and infidelity appear in the story with the characters of Tom, Daisy, Myrtle, Gatsby, and George Wilson.
Though George Wilson is faithful to his wife Myrtle, their relationship is marred by Myrtle's infidelity. She cheats on her husband with Tom.
Tom cheats on Daisy with Myrtle, but this affair is just one of many for Tom. The Buchanan marriage is marked early on by Tom's infidelity. This tendency might seem to excuse Daisy's dalliance with Gatsby. If an actual excuse is failing here, at least Daisy's choice to flirt with the idea of running off with Gatsby is explained by Tom's pattern of infidelity.
Gatsby, for his part, is perfection willing to break up a marriage to get what he wants. He encourages infidelity in Daisy.