But MC doesn't know they're brainwashed either, he just has a feeling something is odd but not what. He knows she just had a ton of sex with the hero, and that all his previous love interests are seemingly aware and into that shared situation. He basically saw her come home from an intense sex session with the hero, and beg for him to fuck her too. "I don't love the hero, I just fuck him! Fuck me too!" basically screams "I just love sex!"