Why do you think that?

I'm in a polygamist relationship, have been for just over five years. No one in the community I know thinks monogamy is bad, I don't think monogamy is bad. Neither monogamy nor polygamy is bad.

It's a personal preference that only matters to the ones making the decisions. As long as you are making the decision, there is no wrong answer

Same here. It's far more often my poly social group tells friends not to enter polyamory. It's right for some people, but not for most people. All the poly people I know have the utmost respect for people who declare themselves monogamous and adhere to it through their behavior. Long monogamous marriages are glorified and celebrated, at least by the people in my poly community.