My x went to counselling once and sat there and basically stated he wasn't going to cut the person out of his life because they were friends. He couldn't see what was wrong with what he'd done. They were in an emotional affair way before it became physical (this time, he'd done it numerous times before with her with past partners which I found out after we totally split). The emotional affair part was way more damaging to us as a couple than the physical was. I could never trust him again & he wasn't prepared to do anything to help rebuild that trust.

We had a lot of other issues (abusive relationship, his undiagnosed narcissistic personality disorder), but the betrayal of trust was the worst by far.

Now, for me, in relationships, trust is the most important thing, if they do anything to erode the trust they are sent packing. This is something they are told about at the start, they know there are consequences to their actions, I am now a strong enough person to follow through.