Which is fine, nothing will ever be perfect. And I'm really not against polyamory at all, I'm just a little against it when suddenly almost every character out of ~100 turns out to be that way, even those where you wouldn't necessarily expect it. In that case I always feel like the writer chickened out because it's always the easiest solution and at that point it honestly starts to become a bit boring.
On the one hand, I agree, the story with Eve as is currently written makes a "polyamorous" outcome very out of character and honestly makes cheating on her with other girls kinda heartbreaking. I mean, I still do it, cause it's a game and she's not a real person, but the game does a good job of making me feel guilty about it.
Furthermore, the problem with polyamory as the universal solution to make a harem happen is that real, healthy polyamory is always open, mutual, and symmetrical. "He's allowed to fuck as many girls as he wants but he's the only boy I'm allowed to touch" is not polyamory. "I'm not really comfortable being in a non-monogamous relationship, but he wants to fuck other girls and he's so wonderful I'd rather put up with it than lose him" is not polyamory. "He's been fucking as many girls as he pleases behind my back, but when I finally found out I wasn't really bothered enough by it to kick him to the curb" is not polyamory.
All of this is just the fantasy of having patriarchal ownership over multiple women in defiance of cultural norms prohibiting such, dressed up in a more appealing coat of paint. Painting this as polyamory is actually doing a lot of harm to the polyamorous community because this is what people think it is, just a justification weird dudes use cause they want a harem. I honestly have more respect for works that are honest about what they are and present the harem as the misogynist power fantasy that it is. (A game like Free Cities for example, the girls in your harem are literally your property. Your MC is 100% the villain of the story.)
But on the other hand, an Eve/Grace threesome scene *would* be really hot. So who can say which one is right?