To hell with all this talk of "rape". MC-kun skippedlet himself be taken completely past first base with that girl in particular and the other one, if I recall correctly--and it's only now he feels violated like an innocent little virgin girl? He may not remember so well, given this fine writing, but he was just about to make out with that girl--with that girl topless--just the other day. He sure as hell didn't see fit to stop her advances, and--just like in this chapter--went along with someone who'd proven herself to have only aggressively lustful inclinations whenever she's around him.
The problem isn't a "rapey" friend, as some people here put it: people are so obsessed with the idea of rape as a "special kind of crime" that they can't see past it whenever it or anything so much as reminiscent of it appears; instead, the problem is a guy who's so milquetoast he thinks absolutely nothing of infidelity. In that regard, MC-kun runs the gamut: from emotional infidelity (the dokidoki moments, the blushing--these are MC-kun contemplating the advances of other women instead of immediately shutting them down) to total infidelity (from his emotions to infidelity with his body, which he has committed several times).
And it's all, no doubt, because of an author who needs to stuff other girls into a story about guy and a girl--who might as well be lovers--roleplaying; this for the sake of appeal to people with a taste for girls like that homewrecker instead of Yukino. Almost exactly the same thing happened in another manga I was once very fond of--but with four girls taking the spotlight from the main girl instead of just one, and the MC of that story doesn't engage in infidelity the way this MC-kun does.