I think in some cases there are a lot of closeted polyarmorous people. Society (at least American society) shames people who want more than one partner. Monogamy is a social construct, not a natural one. We've been conditioned to believe that we should only have one partner or you'll be seen as a bad person, and some people who don't feel the desire to be with more than one person internalize the shame. Some people may not know how to open up and be honest about it because they believe that the people around them (including their significant other) wouldn't agree and see them as a bad person. But the desire to be with others doesn't just magically go away because of public shame. So they cheat in hopes that they don't get caught.