William Dean Howells's 1882 novel A Modern Instance is an extraordinarily influential novel, detailing the rise and fall of romance between protagonists Bartley and Marcia.
During the course of the novel, Bartley starts to have trouble with money and drinking. Earlier, when he helped a young woman named Hannah up into her sleigh, Marcia is enraged, thinking he is intentionally flirting. Later, Bartley tries to help Hannah with her work, but Hannah interprets that interest the same way. This culminates in a fight, Marcia throwing Bartley out, and then later a reconciliation where Bartley admits:
"I am a bad fellow Marcia," he faltered. "You ought to know that. You did right to give me up. I made love to Hannah Morrison; I never promised to marry her, but I made her think that I was fond of her."
(Howells, A Modern Instance, Google Books)
This is Bartley's only specific admission of guilt to the matter. His other affairs may or may not have occurred; Marcia knows this about him and can thus interpret all his actions as revolving around cheating, even as he falls prey to drink and gambling. He is linked to other women, but only by implication does he actually commit adultery. The collapse of their marriage is inevitable as they refuse to let minor slights go unpunished.