You mean polygynous? Because polyandry is a form of polygamy as well.
In theory, polyamory is not polygamy. Polygamy is actually closer to monogamy than it is to polyamory/ENM/RA because it's a
gamos, a closed relationship. In polyamory, all those women can be with other guys. But again, this in theory. They don't consider female hypergamy actually exists so the free love "polycule" usually smoothly arranges itself into polygyny (or
polyandry in the lower side of the spectrum).
I've interacted in the past with a lot of poly people (though I could ascend this way jfl) and they really think they are just making a free and ethical arrangement with all the parts giving their consent. They don't suspect they are taking anything away nor making imbalances because, again, they don't understand/accept hypergamy is there and that sexual selection is inherently discriminatory, thus creating hierarchies. If you told to their egalitarian minds they are generating inequality, they'd fucking glitch with such massive cognitive dissonance.