Well some people get off on men getting raped by other men ┑(￣Д ￣)┍
In more seriousness, I think the authour willingly drew this knowing that his mainly female audience would be appalled by the scene.
Maybe I'm projecting to much but I fell like Lee Uin is infringing typical BL tropes by providing us unusual, and certainly less glamorized, prespectives: the rapist in action allegorized as a snake constricting it's victim, an abuse victim being a naked, and obviously very vulnerable, woman instead of a pseudo-masochist man "who could defend himself if he really wanted to", etc.
It emphasizes the brutal reality of what ukes so often "secretly enjoy" in yaoi.
urghh one thing that makes me extremely uncomfortable in media is seeing women being hit naked. like idk what she did but the fact she was naked when he hit her made my stomach turn. too many freaks get off on shit like that :-//