This is true but only applies if heroine A is dating the protag and then heroine B swoops in and steals the protag
If no one is dating each other then is just a free for all harem, and if a pair becomes exclusive then everyone else gets BSS'd, not NTR
No serious relationship no NTR
and what even is a "serious relationship"?
and thus, we move from literal to tags etc which abstracts the concept.
The same as your usage of the word "harem". If we are being technical about it, it should only apply in cases where the ML is dating/married multiple heroines(e.g DxD). Not simply multiple heroines pinning for a protag that isn't commited to them, which is pretty much the harem genre's "definition"(the trope chick magnet would be be a better fit perhaps, or some other trope i can't think of at the moment, as magnet implies drawing indiscriminately i imagine)
In this case, while no marriage(that is i believe, the legal standard for a serious relationship), "officially" dating(the social "serious relationship"/trope whatever i think),
it is established that we have a pseudo-couple(In chapter 5, both leads expressed a sentiment of a "(near)future" with each other) by plot and setting.
Yes, they are not a couple, but narratively, it is expressed similarly.
By technicality, this series, assuming the common tropes, is neither a harem nor NTR.
But by "genre"(for lack of better word, as i do not think its the best fit for the point i am making as seen in the following explanations in brackets),
it can be both harem(i imagine it won't be long before the tag is added) AND NTR(whether added tag or not), assuming protag "falls" for new girl, as, again, the pseudo couple ship is established
