What I'm getting tired of is the bickering each week, where everyone is trying to police how everyone else is supposed to feel about the story. It's a sensitive topic, and people are going to feel strongly about it.
It should be clear that Ekoda's behavior is inexcusable, from a real-world standpoint. And as we have no reason to think that the laws in-story are any different from the real world, even in-story, Ekoda seems to have broken multiple laws. Whether or not you (as a reader) are hot for teacher, what she did was rape.
However, from an in-story standpoint, I don't think either Ekoda or Shirasawa thinks of it as rape. At least, not yet. Ekoda did ask for his consent, after all. And he gave it. Now, he's still a minor, she holds a position of authority, and she used deception to get him drunk, but I don't think any of that is at the forefront of Shirasawa's thoughts about what happened.
Shirasawa feels betrayed, but at this point, I think that's because he knows Ekoda is only using him as a substitute for Saiga. On the other hand, most of his negative feelings are directed at himself right now. He feels like he's the traitor because he gave in to Ekoda. He betrayed Honjou and the feelings they have for each other and he betrayed his father and the principles of filmmaking that were handed down to him. It's self-inflicted victim blaming.
So, yes, he regrets it, but not because he thinks, "I was raped," but because he thinks, "I was weak." Now we, the readers, have to see where the story goes from there.