Why do "happily" married cheat horror story

Why do men who sat they are happily married cheat? My husband has always been the greatest husband and father. We were going through some terrible things at the time... my health was bad, I had over a dozed big surgeries this last coup,e years. We have three very close family members die in a year. My husband was down dealing with hia alcoholic mother who was dying and had not come to terms with his hard childhood.
😳 my husbands affair partner was a alcoholic psychopath, a term used by her husband. She started out sweet listening to him. then after meeting once offered him a payed ticket to her country for a visit! She offered adventure that soon got out of control insane. My Dh was under a ton of pressure and went. That is where they first had sex... Well attempted. The wholed time they were together he couldn't get hard, even with Viagra! We had never had that problem, we had always had great sex before this. She even started stalking our family holidays in different country's! When he tried again to break it off she brought a prostitute as a surprise! As I have the video my husband couldn't even get started so the girls went at it. Anything to make these guys feel small she did. When the private eye her husband hired came to the door with this info she was laughing how bad the sex was... Obviously, there was no ability for it tobe good when you are being threatened!!! Trying to slip drugs in to booze, showing up in our country and city as a surprise, sent her daughter in school to our city!!! hid cameras during sex and of him crying as she is threatening him that she will to show me these if he ends the affair. She sent them anyways in the end. It turned out to be horrifying. She made up crazy stuff to tell her husband but it turned out she has several guys on the hook! When we finally think she is out of our life she will call or pop up six months to a year later. An alcoholic psychopath confirmed by her husband is terrifying. It is a game to blow up these "nice" guys lives. She picks niaeve nice family guys away for work, takes it slow then once they crossed the line she threatens to ruin their marriage., collects proof, video. Receipts, she has these men scared saying their wife would never take them back with this information, they will lose their kids etc then she will go after them at work. This woman gets off on this. My husband makes the stupidest mistake in his life and we all pay the price! It hurst so much! All I can say is he got so badly burned this won't happen again. Anyone else have a psych who gets off an destroying a whole family???