He thinks he's going to die for some retarded reason. He's jumping to conclusions about the days left thing and is too focused on filling up the "people kissed" bar.
The novel is ntr, but for whatever reason the mangaka decided to change the story and make shizuna's cf a creepy rapist. In the novel mc ntr's him in the same way as the game with her dressed as a gyaru and she even says the same lines.
Same tbh, they sanitized the fuck out of this story. MC hasn't fucked them yet in the manga but at this point in the novel he was fucking all 4 at the same time.