People were cheating long before there was an Internet.
Ashley Madison just made it easier for "like minded" adults to find one another. People who share the same amount of risk tend to trust one another I suppose. They also believe they're emotionally on the same page. Most cheaters aren't looking to replace one relationship with another. Their goal is to compliment what they already have.
There is nothing about the site that is "illegal" although it may be considered immoral. I don't think it should be forced to close.
However I bet there are lots of cheaters praying their information is not publically released! :)