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I think it's a play on the phrase, "to have their cake and eat it, too." Everyone in the movie already had something, but wanted even more. The dad already had a loving wife and had no reason to cheat, but he did anyway. All the other characters seem to have that same problem. That's what I think.

Back in the 18th century, when French Queen Marie Antoinette, was informed that the peasants had no bread she supposedly replied, "Let them eat cake!"

From Wikipedia:

Screenwriter Jayce Bartok is quoted as saying, "The Cake Eaters is a term I grew up with in Pennsylvania. My mom used to use it to describe those who had it made, had their lives mapped out for them, were the most likely to succeed… 'The Cake Eaters.' I thought it was an interesting metaphor for this group of misfits who begin the story searching and longing for love, trying to overcome grief, and through the course of the story… find their 'cake.' They find some love, happiness, peace…."

Rest in peace, Roger Ebert. You were the best.