Hi, well for the strict definition of the word harem:
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in the past in some
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, the
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or other
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of a man, Among Muslims, the group of all women who live under the dependence of a head of household.
So i dont know if the anime or porn genre definition change but for what I understand if you live with women who fuck with you, you have am Harem even if they dont love you and they live and have sex with you for other motives but I think the key thing in Harem is they have to live with you in the end all this genres and definitions tend to be a little ambiguous for example if in a NTR Story can be romance between the MC and FMC even after corruption etc...
In the end is good to have a flexible thinking some definitions and genres can have many meaning depending of the point of view and all have his validity.