Also, it escalated quickly (masturbating together (not knowing what "masturbating" means) -> fucking).
Actually, I may have overemphasized it. She's literally saying, "Let's play together doing perverted stuff" which could just as easily be mutual masturbation. But I couldn't think of a funny and natural way to phrase that.
I toned it down a little. Feel free to replace if anyone an think of something better.
...which means he must be in the clubroom.I hear a violin...I hear Kyousuke's violin...Hey Kyousuke! Let's have sexy time!!I don't want to interrupt his practice... I should leave.
