Being the partner of a cheater is devastating. It's a betrayal of trust concerning the most intimate parts of a monogamous relationship. Not only did you get betrayed, you were tricked. They fooled you. They went behind your back. They lied. It messes with your head. How in the world could somebody who loves you do that to you? It makes you question everything, including yourself.
Along with that, I think a lot of people look for excuses or reasons WHY. I always cringe when I see PTSD used as one of those excuses. Like a sufferer is possessed and has no control over their genitals or brain. I think supporters cling to that idea to make themselves feel better rather than deal with the reality that their partner chose to betray them of their own free will.
Acting on impulses and urges still requires a decision. People may make bad decisions in order to cope or feel better, but they're still making those decisions. This is no different than, say, somebody with combat PTSD having a rage episode and having urges to punch people in the face. If they "couldn't help it" and just punched people, half the world would be knocked the eff out on a regular basis. My vet would probably catch an assault charge every time he drove.