She didn't. He didn't say that her pussy was ugly, or that it looked loose (like how some dudes imply that they've been ran through), he just said it's not like what he imagined, which is perfectly normal for a virgin, especially in japan where most of the porn is censored.
I love how there's so many simps in the comments saying that her fucking the entire friend group might be false when there's no indication whatsoever in this one shot that his memories of the event are unreliable. She fucked them, there's no need to do some mind gymnastics here.
It's obvious that "En-chan", the MC, was shocked and horrified by the sight of Yumi's cunt. And it's equally obvious that he
communicated those feelings to her. Regardless of his words, she understood his reaction, and because she had feelings for him, she was deeply wounded by it.
That all makes sense to me. People can be very sensitive and insecure about their bodies, especially adolescents and perhaps especially young women. So, although En did nothing wrong, he hurt Yumi quite badly. That's just the way it goes...
I agree with you that Yumi probably
did sleep with one or more members of their doujin circle in retaliation. As you say, this is supported by both his narration and her dialogue (see pages 10 & 23), and nothing in the story suggests otherwise.
However... We do, I think, have good reason to take En's account with a HUGE grain of salt. I say this for three reasons:
- The venomous, borderline psychotic misogyny of En's opening narration: "I detest women and their bodies, they're the one thing in this world that I can't stand. Those deformed freaks..." [emphasis mine]
- Though it's easy to miss, Miyu, En's imaginary 2D dreamgirl, narrates the crux of the story. Miyu's narration runs from the exact moment we first meet Yumi on page 8 (coincidence? I think not), to the end of the hotel room encounter on page 22. En's narration picks back up on page 23. According to Miyu, it was not Yumi's promiscuity that broke En: "You poor thing," she says on page 19. "That must have been awful. All of a sudden, she showed you something so filthy..." [my emphasis, again]
- There's no direct indication that En ever cared about Miyu in the first place. He never says he had feelings for her or even that he was attracted to her. That said, he did buy a camera to photograph her and eventually fixated on an imaginary "pure" version of her. From the latter things, we can safely assume she meant something to him. But it's hard to say what.
The most reasonable interpretation, I think, is that En was, from the beginning, so psychologically fragile and obsessed with the idealized perfection of 2D beauty, that the mere sight of a real woman's cunt literally shattered his mind. Everything after that, from Yumi's cruel words to her possible promiscuity, was incidental.
I believe that the real point of the story is delivered by Yumi on pages 28-29: "Men and women can't avoid hurting each other! That's life!" It's a simple statement, but one that draws a clear line between Yumi, who grew and got on with her life after a bad experience, and En, who simply could not.
P.S. I don't think sleeping around in the wake of a bad romantic experience is unusual or inherently immoral. Even if Yumi did have sex with the other members of the circle to "get back at" En, that doesn't necessarily make her a villain. She was a young person making some poor but very common and understandable decisions.