For my part, I really fucking hate NTR. I'm not going to go on a moral crusade against it, demanding it never exist, but for some people it is... emotionally disturbing. Being cheated on isn't pleasant. And for the "Why are you associating yourself with the cuck, bro?" crowd, "If she'll cheat with you, she'll cheat on you." The idea that your dick is so great it'll make her a slave to it over all others? Well, that can be your fantasy.
But in this game, I'm not even especially bothered by what's being termed as NTR in this case. It's barely appropriate to apply the term because the whole game is about everyone being sluts, the MC likely being the former ruler of a legendary nation of sluts, your job is to be a slut, Rika's former job involved her being a slut... this isn't exactly a new context being introduced. My issues with her quest is that it's discordant with the rest of the game thematically. For a start, the MC shouldn't have a problem with Rika wanting to get fucked by monsters, but for some reason he does. The player is given a choice about asking Rika to remain monogamous when I don't see why that'd be a consideration in the setting, but it's given so the player rolls with it... and it doesn't matter. It doesn't matter either way. If we say we want her to be exclusive, she goes out and fucks monsters anyway. If we say she doesn't have to be, she lies to you about doing so and acts like it's something to hide, and then the MC gets upset anyway. This drama is both unnecessary and unavoidable, and none of it should exist in the first place. There are other ways to reach all the same plot points.
First option, if necessary - Rika sharing content. Yes or no, if no, you don't see any of it. I personally don't think this choice even fits with the setting, but whatever.
Assuming sharing is accepted, second option - Rika's sexual frustration. She can be enjoying sex with you and potentially being a partner in the breeding pit, but you get the sense that she's not quite as... mellow as you might want. Then you chance upon her getting dicked by a wild wolfman at night, and she confesses that the size and thrill just scratches that itch she can't otherwise be rid of. No drama from the MC, she's a little embarrassed at being caught and nervous the MC will consider it a slight, but there's not any reason to. The MC is a chill character in every other scene. But, MC does want to personally fix this problem for Rika, so the transformation questline. I personally think it'd be more interesting to have this be something greater than just a straightforward lycanthropy thing, which is a bit dull IMO.
Same plot points, same scenes, same art, no drama. It's all about the framing of what happens. The entire questline involves massive tonal and thematic shifts from the rest of the established game, and then on top of that the player is misled by the offered choices in both potential ways. None of this was necessary.