I know that I'm in the minority here but I don't see the ML as a mistake. When my H was deep in the A, he was telling OW that even though we were living together that we didn't ML. I know that for my H, ML is his most important way of connecting. Of course, we ML and it was confusing to him.
But when he would say that it was a mistake to ML with me, I would say that "The mistake is the A. We are H/W. This is what's right. You're feeling this way b/c you know the A is wrong. We are what's right. OW is the mistake."
It definitely worked. I had myself tested and continued to ML when the opportunity was present. It was the best way for me to keep the connection alive when he was trying his best to kill it.
I didn't expose to OW right away. It was several months in when I had enough and called her. It blew up in his face. I told her that he had been telling me that we would try again and were ML all along. Of course, he had told her that we didn't. She kicked him to the curb the next day. My H's OW was single so there wasn't anyone else to expose to. However, if she had been married, I would have exposed to OW H right way.
Read Dr Phil's, Relationship Rescue. It works well with DB. It helped me have more confidence and know how to say what I needed to say when the times were right.