I thought to add another point of view. I'm generally a very trusting person, but if I were to start a relationship with somebody who was cheating on somebody else (don't think I would but never say never), I would find it hard to trust them. How do you know he's telling the truth about all the stuff going on in his marriage? How do you know you're not just one of the many women he lies to?
I'm also the kind of person that in the end I cannot take it when people don't help themselves. Even if I understood the reasons somebody stays in their (dead/dysfunctional) relationship, I couldn't be there to support them in that decision if it's making them miserable. My empathy will run out at some point. Will yours?