Not strictly related, but I really want one of the big brain people in this thread to explain to me the difference between a harem and a poly relationship, considering how people rejected marking Nanami's relationships as a harem.
Polyamorous relationships can take on different - as I like to call them - topologies, and do not have to be centered around one person, like Nanami's setup. One person with multiple unrelated partners is one extreme with the other being polyfidelity where everyone is committed to everyone. The first one may resemble a "harem" but I hope you can see how the latter doesn't have much in common with it.
As for why Nanami in particular doesn't have a "harem" : she is romantically committed to all her partners and every partner knows about the others. This is not an open relationship or casual hookups that I would describe what happens in the harem genre as. In harem manga there are multiple suitors all competing for the MC's attention and wanting to have them all to themselves and the MC does not take a stance on the nature of this relationship.
And then there's the origin of the word, where harems were comprised of women having sex with a noble and looking pretty in his estate in exchange for luxury and a high social standing. More prostitution than romance