That’s kind of messed up. With straight harems, at least most of the members aren’t attracted to each other and only the person at the center of the harem.
My lesbian princess character in
I Was A Hero In Another World But My Dog-Girl Slave Swapped Places With Me And Now I Can’t Disobey Her is cool with her harem sleeping with each other, as long as they prioritize her over their other lovers, and don't get pregnant.
Other lesbian harem-situation fiction I've read, they're generally a little less possessive as well? But obviously it'll vary between works.
...also, I'd say a situation in which one person handles the emotional, romantic, and sexual desires of a large number is way more messed up than a more equal polycule in a variety of relationships, as a relationship with one central person and everyone else focused on them, the people focused on them are more likely to get neglected, and jealous and competitive with each other.
...but that's neither here nor there for the main point of if the tag should exist or not, there's lots of messed up situations allowed here. Messed upness of the situation is kind of a tangent from well, just tagging things well so people can decide what they wanna read and what they don't.
Maybe, it would be better to fix the description of "Polygamy"? "-gamy" is about marriage, not gender. It can be either polygyny, or polyandry. While polyamory is not about marriage and family (well... legally recognisable family, at least).
This'd be good in theory?
I feel like polygamy is assumed to be something else on here, and thus it'd be way more of a mess than just making a new tag-- but I could be wrong about this.