I don't see where drinking/drugging is accepted?
In the end, drinking/drugging/cheating is all...
Meaning that we seem to understand why PTSD sufferers turn to drugs or alcohol or workaholic or gambling behaviors as unhealthy coping mechanisms. But if someone cheats as an unhealthy coping mechanism we call them an asshole for doing this. We don't label the drug addicts or alcoholics as assholes for coping in this way. We pity them, we urge them to get help, etc. But a cheater is labeled an asshole and lowlife and "how could you do that" unless the PTSD is from sexual abuse, then we seem to understand.
I am NOT excusing cheating as acceptable or healthy or any of those things. It is an emotionally devastating and cruel behavior. However, what is not coming through here is that the cheating is a coping method for killing the pain and putting up walls to intimacy due to the PTSD.
Everyone has a coping method, like Caligirl said. This is needs to be recognized as such instead of just told "you're an asshole". How does that help the sufferer get better and stop this behavior? They already feel like trash inside and labeling the person as trash only reinforces the behavior as correct in their mind. They need to be told they are valuable and too good for this kind of stuff. They need help just like a drug addict or workaholic.