Generally speaking, in classic harem games, if you reject a love interest, she either becomes a friend or simply disappears from relevant scenes involving her, rather than the game creating scenarios where the protagonist peeps on her having sex with other men and then jerks off to it.
Maybe you're playing a non-classic harem game, like “Between Worlds”, which has NTR and harem romance routes. If you reject her or suggest that your love interest find another man, in the sex scenes between your love interest and other men, you appear as a voyeur and a cuckold fetishist.
“Between Worlds” creates NTR content in exactly this way.
https://f95zone.to/threads/between-worlds-v0-1-9-pi-beta-ex-roleplayer.129383/
Interestingly, “Between Worlds” does not use both the harem and NTR tags at the same time, even though it actually has harem romance routes.
The statement from "Between Worlds" is clear and unambiguous; all actions are optional and will not cause NTR controversy.
Sharing and Netorare routes are separate and optional. No kinks are forced, and routes are set somewhere in between the vn by simple questions, and once set, they flow in that direction.
Between Worlds is based on a fantasy-based theme where the society has a polyamorous culture. Totally consensual and, by choice, non-monogamous. Mc can have multiple partners, and so can their Li.