But if the ones that do cheat blame it on something (anything) except themselves
Yes, adults are responsible for their actions, though that does not change that an underlying reason may be helping their decision along. It isn't to blame something else, but more like a legal stance, where mitigating circumstances are present. I think
@caligirl03 has many facets quite accurate, with the exception I would not underly combat vet as opposed to the more accurate reason of PTSD (mental health):
lacking healthy coping skills after all they've experienced
not knowing where to turn for help or simply not wanting to get it, instead resort to more destructive ways of helping numb the pain-whether drugs, alcohol, or meaningless sex- even if it's just to forget it all for even a moment.
There is another facet not mentioned here, and that is that PTSD sufferers, period, feel a range of negative emotion towards current partners where those emotions are based on basically failing them, especially those where the relationship was pre-PTSD and then PTSD came along. One method is for them to cheat on their partner as a failsafe method to exit the relationship and no longer harbour the guilt, remorse, that they have convinced themselves off, or may be quite realistic based on things the partner says to them (comparing pre and present relationship status), and provide them some relief that they're doing the right thing for their partners overall longevity by getting away from them.
Its basically a moralistic underlying belief, performed in a hurtful, abusive, un-moralistic manner. They believe they're doing the best thing for the person they love, by finding something to distance themselves from their partner with, ensuring they won't take them back.
There is quite a psychological manner in all of this, which is often found by psychologists treating the problem.