This is a new relationship, metaphorically. But the past and history is there. This is not a relationship with some new guy. That would be so easy. Oh my heck, so dang easy.
I do get where you are coming from - although I am not so sure that a new r is really easier. Depends on the people!
I think the problem is not simply what the MLCer 'did' while in the throes of MLC, but the beliefs underlying this, that it was somehow OK to do those things while still married because in their heads they considered they were not.
Trust is about behaviours - what people actually do, not what they say, Behaviour doesn't lie.
The truth is that this person thought what they did was OK at the time. That they were entitled to act in that way, without concern for the feelings and welfare of others. Now they may have changed their mind, but what about their underlying beliefs?