it doesnt but you dont have to worry about monogamy-only rules like dont do this and dont do that, which is exclusive to monogamy. which makes it objectively worse since you dont have to worry about that in other relationships.
All of the others - short of anarchy - also have their rules. For example, polyfidelity is nearly identical to monoamory except you now have to consider the dos and don’ts of both of your partners making it objectively more complicated than monoamory.
I don’t see how any of that is objective aside from it being the most commonly accepted, or “pushed” in your words.
A successful two person relationship requires a significant amount of compromise and sacrifice. How does having more partners lessen that?
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All of the others - short of anarchy - also have their rules. For example, polyfidelity is nearly identical to monoamory except you now have to consider the dos and don’ts of both of your partners making it objectively more complicated than monoamory.