I would argue that western society is already non-monogamous, since in many countries about half of the couples divorce. This results in "serial-monogamy", that is certainly non-monogamous.

FWIW, I think most reasonable people would call this moving the goal posts.

Most people in western societies at any given time are engaged in monogamous relationships. In fact, it's a crime in the U.S. to get married when you're already married to another person. Western governments don't recognize polygamous arrangements for tax purposes, etc.