This is where you can see the evil of a spineless male MC. His own little sister clings to him for protection from a fate she ardently wishes to avoid--i.e., being used as #1's sex toy. Yes, that's exactly it, despite his saying it was "love at first sight"; he was also the one saying that he was getting <i>tired</i> of all the women he was ploughing. All he likely sees in Mahiru is a pussy that's different from the norm.
Whether that's true or not, Mahiru does not want that....and yet Reito, in all his milquetoast, effete passivity, barely puts up a defense of the girl--even while she's next to him, obviously seeking his support and protection. There's a limit to how agreeable you can be; once you pass it, like Reito has, you not only become a doormat, you also become malicious.