Also, there's a large number of people in society that are more concerned with image than substance. Sigh.
At least with cheating--in their minds---you can appear to everyone else as still doing Monogamy and thus part of mainstream society. As opposed to Polyamory which is a "fringe lifestyle" in their view.
(Kinda like people still members of a church or religion ,even though they never go and deep down probably don't hold those beliefs anymore or follow the basic tenets.)
This, I have seen personally, especially big as a gay man who's poly, and is frowned down on by fellow gay men even, though I know
they are all fooling around on their boyfriends. But as long as they "appear" to the outside as the perfect happy monogamous long term couple fighting for "marriage equality", etc, then that's all that's important.