Humans are not monogamous. A monogamous species would not feel sexual attraction and desire for people who are not their mate or a potential mate. The mere fact that we want to fuck everyone is proof, and insisting that humans are monogamous despite the available data showing someone has other partners in a huge portion of all long term relationships sounds a lot like denial.
Human "monogamy" is a common social convention, not a biological feature of humanity. DUH.