I believe that the vast majority of people that are unfaithful are monogamous in their beliefs. The ones who are not monogamous in their beliefs either live in poly relationships or consensual non-monogamous relationships, or they have divorced. If it's very bad, then people don't stay married these days in the West. They can be married and have their family, but they want something else - they want something that they don't have in their lives, or simply to be someone that isn't who they are in the context of their marriage.