Husband facilitated and fully intended on having sex with his ex girlfriend

I found the messages, he doesn't deny it, has been so cold about it, no remorse, no apology, nothing, just carried on as if nothing had happened, I am utterly crushed.

He said it was my fault because I locked him out.

There's a history, our marriage has been difficult for the last 2 years due to his mental health and addiction, I've stood by him through it all, taking constant abuse and critism, advocating and fighting his corner, for him to throw it all back in my face with the ultimate betrayal.

We had an amazing first 11 years a dream life, he was the perfect husband, I literally don't understand how we have gotten to this, he went AWOL for over 12 hours, said to our year old son that he is no longer his dad and drove off, this was really unusual behaviour on his part, naturally everyone was concerned and when he wasn't answering his phone to anyone I decided to call the police to consider a safe and well check, they found him so that was good, the next day something was niggling me about his disappearance so I checked his phone and saw all that I need for full confirmation, he didn't even seem phased when I told him that I knew the truth, I feel so sick and utterly devastated - He had contacted his ex girlfriend from 14 years ago, completely out of the blue, they met up and went drinking, they went back to her house and she took her clothes off and was all over him, he "apparently" said no and told her to put her clothes back on and then left, he said that nothing happened and that I should just drop it - The messages stated that she was so "close to getting it" but he had promised to look after her and having sex with her whilst she was drunk wouldn't be doing what he promised and that he will "do it when she is sober" he then consistently messaged pushing to go back the next day, the girl had some sense and said no, he continued to pursue it then came home to me, STILL maintaining that it was my fault because I locked him out (There is a DVPO in place) and that we were over because of that so it doesn't matter.

I have considered whether this is in my opinion cheating, I have spent the last 3 days at odds with myself over it, even if nothing actually happened that night he fully intended on following through with it had she agreed.

What are your thoughts?