Betrayal from fiance who has ADHD

I'm engaged to be married next year.  This past Sunday I discovered that my fiance responded to two personal ads on Craigslist a week ago today. One was for a woman and the other was for a man who was looking to experiment for the first time. In looking over his email, neither of the people responded to him so he didn't get the chance to act on it.  He has said that he didn't plan to act on it.  He just got a thrill from sending the emails. 

He was diagnosed with ADHD when he was younger and now, at age 24, he's not taking medication (no insurance) so I wonder if that plays a role. It's bad enough that my research of ADHD revealed that sexual addiction is a common behavior in people with untreated ADHD. I've tried my hardest to be patient with his condition, but today's finding makes me not care about his condition, for the first time in my relationship and that concerns me. 

When confronted, my fiance said it was all just a "fantasy" and that he got a rush from it but never intended to act on it. I told him the minute he hit the send button, he "acted" on it. I don't believe it. He has always had an eye for online pornography, which honestly didn't bother me. But I felt he crossed the line when he went from looking at photos/videos of porn stars (that the whole world has the chance to ogle), to contacting complete strangers in our area for random sex hook-ups.

Since this occurred, he has had 3 counseling sessions with a member/therapist of his Mom's church.  This is a man who doesn't beat around the bush and is very direct and doesn't accept excuses.  My fiance grew up with parents who never held him accountable for his actions and I am the first person in his life who has.

Initially, he struggled to see how his behavior constitutes cheating. He said he has never physically or emotionally cheated while we've been together.  I believe that he hasn't in the past, but I am having such a hard time believing it for our future.  I guess, in my mind, it would hurt to not have him in my life, but over time, I would rather be alone, than invest my time in someone who can't be faithful to me.

I think (ah, the Gemini motto), that what hurts is that he knows that my last serious relationship ended (by me) when I discovered my boyfriend was having sex with his coworkers and lying to me about it. He knew how I felt about cheating and that it's the one area I won't waffle on.

I believe that he hasn't had sex with someone else, but what he did is still cheating in my eyes and my pride (and rational side?) makes me want to show him that I am serious. I'm not married yet (this would have been my 2nd), but we do have an infant together.

I apologize for rambling. I guess I just wanted to get my thoughts out there. We're scheduled to move into a new townhouse together on Thanksgiving Day. I told him today that it would be best for him to look for other places to live. We'll share custody of our daughter, but I wouldn't be comfortable living in the same house with him while I dealt with this.

The expected signs of remorse have been present since Sunday. He's cried, begged, pleaded, made dinner, blah, blah, blah, blah, blah, but I feel like my heart has already hardened. I didn't even allow myself to cry until late yesterday and I couldn't stop for over an hour.  I guess I'm still in shock. In the past when we had a disagreement, despite the disagreement, I still felt love when I looked at him. Suday was the first time I didn't have that feeling.

I've browsed other chat forums and posted something similar and I immediately got negative responses that chatised me for even thinking that ADHD could have been part of the problem.  ADHD doesn't excuse his behavior but I guess what I need is feedback from people who have TRUE experience with ADHD, rather than just jumping to the conclusion that it's effects are a myth.  I don't know enough about ADHD to know what he goes through on a daily basis emotionally and physically.  He has trouble explaining it to me.  If I were to stay with him, I feel I have no choice, but to find out more in an effort to survive this.