Spoiler (highlight to view)
Deceiving someone he's feeling up all the time, while trying to get in bed with another person (also under false pretenses to that person), which he later succeeds in doing. In the earlier chapters you can say he didn't know any better. However even after finding out the truth, he decides to take the path that benefits himself the most.
You feel so bad for Yuki Nomura and Oozora as Spoiler (highlight to view)
the story progresses...
This is not a happy manga, but i guess it's a bit more realistic because afterall there are bad people in the world who would behave like the main character does. 
He looks at every female he sees with lecherous eyes, and has no problem masturbating to the thought of
Spoiler (highlight to view)
Oozora who he pretty much raped through deception.
I just can't believe that Yuki's cousin who claimed to care about Yuki let this deception carry on. The main character's best friend Hidepon also deserved more.Spoiler (highlight to view)
Due to the ending... >:-( I feel like making a doujinshi where it turns out Yuki and Oozora actually only pretended to like the main character after discovering the truth, and then after graduation get together after getting revenge on him.
I mean, if he grew gradually, that would have been cool. But for him, it was just *click*, complete 180. Made the story feel incredibly artificial. 