There are powerful entities, including religions and country laws, that either make life easier for people pratcicing the monogamous relationship style or just outright force that style on masses of people. This force spawns resistance, and the negative view you mention is an expression of this resistance.
> There are powerful entities, including religions and country laws, that [...] just outright force that style on masses of people.
Systemic, indoctrinated and even toxic monogamy, perpetuated by people and society at large.
You've got people and relationships that are so harmed by strict heteronormativity and its related monogamy that "men and women can't be friends" and "you can't say that lady's cute because that's cheating" and pornography is adultery.
It's little microaggressions and requirements of conformity that systematize and enforce monogamy in little ways.
Imagine the same system enforcing poly. You’d have mandatory partner sharing to those in power, all sorts of requirements and gate keeping for who could and could not share, or be part of various cliques, etc.
For sure, anyone pretty or desirable would be expected to be poly and like it or else, etc.
The reality is that system would fly apart even faster than monogamous systems since there is no meaningful way to ‘opt out’ and protect oneself, which is why you don’t see it in any large scale societies.