What is considered cheating? When should I stop forgiving?

Here's my story I have been with my fiance for over 2 years. When I met him I was still married to my ex and working on finalizing my divorce. Throughout the 2 years I have caught him posting himself on Craigslist in casual encounters. I have not caught him in an ual act but I have found emails of him with other females. Yesterday I went into his email and found 4 emails he had deleted but had not been removed from the Trash bin. One of them was to a .It was a dude because the picture on the email was of his behind with his xmas ornament. When I coned my fiance he stated that I was all a mistake and he did not do anything. He tried to make me believe it was the ual emailing him, in which it was the other way around. Then he stated that he thought it was a female. At the end of the day he said it was my fault because my divorce was not finalized. What should I do?