The heat of the summer seems to make everyone behave in an illicit manner. There is more drinking and gambling at Gatsby's parties, Daisy has always felt a certain heat for Gatsby. She is confused by her feelings, since she is a married woman. Since her husband is cheating on her, she looks to Gatsby as her fantasy man. He still loves her.
There is a molten passion between Daisy and Gatsby. Tom, Daisy's husband is already having an affair with Myrtle, so there is the heat, that is unbearable and all the adulterous romance between assorted characters heating up in the novel.
The physical heat of the sweltering last day of summer is no more intense than the fiery emotions—temper, passion, jealousy—of the characters.