Harem = 1 person is loved by many. Poly = a group of people love each other.
Exactly. In my own trio, the two women have a relationship between each other as well as with the guy.
The line can get a bit blurry if you have a story with a growing group, which is much, much harder to pull off and do well.
Honestly, even if it's all-group love, it's hard to maintain a growing group and keep it interesting.
There's one reverse-harem idea that I have that would be stable once formed, because of the reason it's forming. The nearby elven kingdom (there's more than one) in my setting is technically a queendom, and before taking the throne the heiress to the throne should have formed her council, which will be men who will also be her consorts.
So the story would focus on a princess who hasn't started forming her council yet, but a few years ago her mother started hinting about wanting to abdicate/retire in the next century or so, and said princess is over 200 so it's probably about time she go and sort this out. Oh, she has plenty of applicants already, but the ones who might make decent councilors she doesn't want in her bed, and the ones whom she might want in her bed (or has already had in her bed) aren't great councilor material in her opinion.
She's legally required to have a minimum of three at the time she is coronated, but there's more than three areas of expertise one might want covered. Someone like a paladin could cover both military advice and religious advice, but do each less well than an experienced military knight and a priest. Well, the choice is hers, if she can find the people she wants and convince them to agree.
The only bloodline that matters is hers, as only her daughter can inherit. So just like she does not know which of her mother's councilors is her father, neither will her children know which one is their father.
Now, how to balance practical decisions for councilors with men whom she actually wants in her bed? (which is the crux of this story)
Loyalty is at least a non-issue: To be affirmed as one of her councilors, a candidate has to swear an oath of loyalty to her and the kingdom. Said oath acts as both a divinely-powered sincerity check and a binding magical oath. On the flip side, a councilor *may* have an additional relationship, with her consent and her approval of the partner (because of potential influence on the throne via even a loyal councilor).