Throughout the game, the MC has struggled with the glamour — the idea that others act uncontrollably and possibly against what their will might normally be. He has a really hard time with thinking all of the women are with him because they're forced to be. So when she suggests she wants to be his sex doll, he feels like it's in the same vein: she's subject to any of his desires and has no will of her own. What he doesn't realize is that she normally can't feel anything anyway, so being a sex doll lets her believe she has purpose and, in her own way, feel something. Not sure if it's actually working, but that's her reason for wanting to do so. The MC just doesn't like it, like I said above.
