Except she does, her decision made here>>>
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<<<determines the trajectory of the whole rest of the game.
Yes, like I said a few posts up, there are some exceptions. This would be one of them. However you only get this choice if Ethan didn't interject earlier when Nat talked to Darrel outside, so again it was Ethans choice who ultimately led to this choice now if I remember correctly.
But regardless of that, the further you get into the mutual cuckold path these choices happens less and less, and her inner thoughts and feelings are almost never brought to light, unless she is directly indulging in Ethans desires within a scene.
Also this choice, like you said, determines the trajectory for the rest of the game. I would like more choices from her which doesn't necessarilly change the path of the game, but more so reveal her perspective and her character shift from being in this cuckold relationship. What is she really thinking, what are some of her own desires going forward, will she push Ethan to try something more daring? If her thoughts and ideas going forward are still just gonna be influenced by Ethans desires, i'd say that's a lack of character development and she is essentially the same as when they started. The only thing different is the sexual acts themselves. She is more comfortable with them, but her overall personality and thoughts remain the same outside of sex.
Which is even more unrealistic since this idea was Ethans to begin with and she ended up liking it, of course that would change her views and feelings moving foward. It's like she discovered a side to herself she didn't know existed. Unless she is literally only doing this cuckold thing because Ethan wants it...which is obviously not the case, she likes it too. And that change in her should be addressed in her thoughts and ideas going forward, and not just in sex scenes but also in casual everyday conversations between her and Ethan.