Speaking as an "ex" and current "silent supporter". our relationship lasted 4 years, and it was great (or so I thought). One day, he came to me and said "he needed to do things on his own". This stemmed from a disagreement we had earlier that morning because I told him I couldn't run our household by myself (things you never say to a PTSD sufferer 101) but in my defense, this was pre diagnosis.
I found out as our relationship ended that sufferers often revert to "fight or flight" response.
In my experience, Luke felt/determined he could no longer care for me the way he thought he should, (this is a common one as I have learned) and I "knew too much", I too was starting to have suspicions, asking questions about one person in particular (a fellow classmate I didn't trust from the get go), I respected Luke's privacy, and made my feelings clear but I let him know I supported his decision, and that I trusted him!
I hate to sound like this but I feel a sufferer will always have a plan "B" and probably "C" and "D" as well. As long as everything is fine, there is no threat but if the threat becomes heightened, it's time to go.
If he is keeping in touch with these women and not communicating openly with you about it, trust that he is hiding other things as well, your only scratching the surface! But also know, he can't necessarily help it and prepare yourself for what may happen should you decide to put your foot down.
Luke started his new relationship "officially" just five days after I left our home, it was the first and last time he ever put his hands on me, I was not sticking around. He refused treatment, was discharged from a hospital, on some pretty heavy meds, and that was the last time we ever stood in the same room together. That was 10 months ago!
I have met "in person" three other PTSD supporters who have similar stories, one lady had a gun pulled on her, two others found letters, emails and texts from other women where the husbands were complaining about their wives or not disclosing they were even married!
If what you are describing isn't PTSD, I would dare say it's at least not uncommon behavior for somebody with PTSD, whether that is behavioral or not.
I am no MD, but I have seen that death stare and watched the man I love look at me as if he were going to kill me for breathing, I know he is capable of cheating and not thinking twice about it, and I know that there are probably others who can relate to your story as well!!!
I hope this helps!