Wait a minnnute, if they just had sex, she would definitely know it, just by the sensations and uhm, foreign material, inside her body; she'd also smell his scent all over her and likely wonder why she's all sweaty. Not to mention that her clothes would be all soiled. Has the author had no sexual experience? It did not show him cleaning her up after they fucked, and they were still in the dungeon after all. Perhaps the translation of what the bell's effects were was off, because stating that the target would be "apathetic" does not match how Harumachi reacted at all; she welcomed the sex. Telling an apathetic person that you love them should not garner really any response at all. He also didn't state to her that she would forget that they had sex after they did it, which is what I assume he was trying to hide during that 15 minutes. I guess him saying "...we can exchange love like this..." is supposed to make her fall in love with him, but in her apathetic state, would it really instill such a notion so strongly?