Chatting sexy messages, flirting online - same as cheating?

My fiance and I have been together for about 3-1/2 years and I have had trust issues almost from the very beginning because of his behavior.  When we were dating (first 3-6 months), he would flirt with bartenders, google-eye other women while we were out, etc.   I pretty much felt invisible much of the time.   I put my foot down with that and then found out he was texting (probably sexting) other women.  Since then, I have found emails and chat messages where he is calling other women sexy, asking if they are naked, etc.  I was devastated and when I confronted him, he said he was just joking around, that's how he jokes.  Right.

He has been so different in the last four months.  I finally felt I truly trusted him again and our relationship had reached a much different, higher plateau.  His behavior is completely different from the beginning, very respectful, attentive.  I felt content for the first time in our relationship.  

Unfortunately last week, I found a chat mail to his first love in high school.  He saw her about 1-1/2 - 2 years ago and proceeded to tell her in the email how he wished he would have brought her upstairs for a "quickie" that day.  This is probably the fourth or fifth time he had done this type of thing, knowing how much it hurt me before. Stupid question...Do you think that he can be trusted?....ever?  He says he's never cheated on me, and I had to explain to him that this IS cheating, although a different type.  Am I right?  I'm tired of being disrespected this way.  Thank you!