Hating NTR but liking harem

Honestly, it depends on the novel you're reading. Some harems have the MC NTRing other characters to add to their harem. Some can be completely free of NTR, but at the same time, the guy is not faithful to an extent. Nowadays, having more than one significant other is frowned upon, unless you're in another culture or just don't give a damn.

Usually, in JP novels, the MC can get a harem due to just the genre. However, generally, people hate those due to how one dimensional they think the women are. However, if you don't care about that much, go for the generic harem novel, junk food is still junk food. Some can just be extremely dense and not appreciate them for a long ass time. Those either end with open endings or sticking to one girl.

In CN, some of them usually have the MC NTRing some person's fiance or some girl who's "never interacted with another boy/man before" which is kind of funny. I mean, seriously, you're with another guy, your father and/or brothers usually in those novels. You'd expect some sort of familiarity with them by the time you're a teenager. However, there are some others where the MC somehow gets the most "beautiful" woman in the city due to... luck? Well, in the end, CN cultivation is generally a "Power" fantasy. Although cultivation makes the women powerful too... which begs the question of why they "feel weak" when the MC is weaker than them sometimes.

In the end, if you don't want harem or NTR, just make it so you filter them out of your search.