No. That's how they are advertised to you. Here's how Ashley Madison is actually marketed/advertised.
1. There are those garish "Life Is Short - Have An Affair!" ads. They are not really meant to bring in members, nor are they want prompt most people to sign up. Their entire purpose is to let people see that they are a big site with a lot of real members and that if you are married it's a place you can go.
2. Then there is affiliate marketing. This is what gets most people to sign up and if you are the type of person who might get offended by a non mono relationship you've probably never seen it. What happens is that AM pays a commission to a site that can get people to sign up for it. What these sites do is target the real demographic - poly people, cuckold relationship people, gay people in beard marriages.
What the sites do is offer helpful information in general for people new to these types of situations. Because people in these situations need to find out how to meet people to date outside normal dating channels. The sites that offer solid information are very popular because people need this information. Then, in the articles themselves it is pointed out that Ashely Madison can help them. Another variation of this is that the site will have erotica based on these scenarios (because people may choose to read erotica before they look for real advice) and then have customized Ashley Madison ads on the pages of the site.
Now, if you are not one of the people who needs Ashley Madison for these non cheating reasons, there it is unlikely you've seen those affiliate ads, which are the most common ads for the site, becuase you haven't gone to pages that would provide you with those links.
This is why you may think that most of the people on the site are cheaters and that is who the site is for, but you'd be wrong. The truth is that it's marketed much more extensively than that.