I almost could have written your post. I did confront him about a cheaters site I knew he was browsing months ago and he said he just clicked on a pop up ad & browsed around out of curiosity. We did not argue about it, we just discussed it; he claimed he would never cheat on me. I mostly believed him at the time & we had a couple of marriage counselling sessions since, and I have gone on anti-depressant/anti-anxiety meds which are improving my mood & motivation. I thought things were improving between us.
I've noticed for the past couple of weeks, he's been spending a lot of time on his computer. He has a list of passwords on his desktop, so I have access to his email account. Today I found that he very recently made a profile on Ashley Madison looking for "short term" relationships. He's got messages from 2 women so far, but hasn't responded to them. I've printed out his profile and also one other I found where he signed up for a different dating site, but no profile yet. I just don't know what to do at this point. I feel bad for going through his emails, but he knows I have the passwords, so obviously isn't being very discreet.
Sometimes I think behaviour like this is just a cry for help because he has problems bringing up things that are bothering him. He's been pretty indiscreet about viewing porn sites, Googling things like "fridgid wife", "wife has no sex drive", etc. He also writes out letters or thoughts and "hides" them in places I will likely come across them.
We are planning to go back to marriage counselling soon, supposed to plan a family vacation before then though. I think I will wait to confront him about the Ashley Madison website for now and see if he actually responds to the women and what happens. I know, I'm spying on him, but it's his choice if he cheats & if he has already made the decision to cheat, he's already cheated in my eyes. I don't know what advice to give you; I just thought I'd chime in because I'm in the same situation and let you know what I'm doing about it.