This MC--I forgot his name again--is truly stupid. He's read multiple sex stories with a girl who said that she'd only act them out with him, and he can't figure out the nature of what he's reading to another girl. So, then, that proves that he learned nothing. I'm willing to bet that Yuki slapped him because of his constant cavorting with other girls--to the point where he was actually engaged in sexual activity with one of them. If it was for that, he'd have deserved it.
That said, this is the second time I've encountered a pretty, busty, bespectacled, gentle/sweet female lead with backstabbing "friends". Doesn't this girl know that the MC is involved with Yuki? Yuki might be best served cutting all three off, at this point.