Surprised to see so many of the comments over the last few chapters being upset about the direction things went, rather than simply admiring that it's actually great storytelling depicting pretty realistic emotional complexity.
Was Sensei wrong for taking advantage of Shirasawa? Yes, by any means it's statutory rape. Not intending to "brag" whatsoever but I experienced this type of thing when I was a 16 year old male with a woman 10 years my senior, it was a messed up situation that left emotional wounds but these things happen and I'm fine now. As such I'd like to think I can speak to the issues here with a little bit of authority even if it's just one person's opinion:
Those saying Sensei is simply a predator who outright forcefully raped him for her own self-indulgence aren't paying attention to the story. The situation unfolded as it did because both Sensei & Shirasawa have character flaws that were being slowly explored/set up and finally came to a head leading to what transpired. Sensei had always hid underneath a mask because of the way she was treated based off appearance alone, and Saiga & Shirasawa as directors were the only ones to "see" her true self underneath through directing talent. She admired Saiga's directing talent and ability to connect/see the real her, but ultimately he rejected her value as an actress which left her scarred since she deeply trusted his ability to see the truth about people. Deep emotional wounds like that can lead to people engaging in problematic & manipulative behavior, especially against vulnerable individuals or where a power dynamic is unbalanced.
Along comes the young & talented Shirasawa who displays that same ability to peak underneath her mask and draw out her true self, but this time it's different because he sees the beauty in her ability and admires her for it rather than rejecting her. While she does compared him to Saiga, it's not exactly that she's using Shirasawa as a 'replacement' for him as Shirasawa implies in his outburst. Rather, she longed for this deeper interpersonal "director-actress" relationship which crosses boundaries and previously did not end the way she wanted. With Shirasawa she saw her chance to relive the opportunity with a new director, not as a substitute for Saiga, but with one who actually drew out her true self to an even further extent than him: She saved scenes from the hot springs filming on her phone for 'personal fulfillment' saying that they were her favorite even above everything she did with Saiga. All that being said, she ultimately does "play dirty" by giving him alcohol and making very strong physical approaches to seal the deal with Shirasawa, because she wants control of the situation to go her way this time and not relive the past emotional trauma. While he did not resist, and in fact reciprocated, it's still not okay no matter how real her feelings were.
Now onto Shirasawa's character flaw which allowed this situation to transpire even though he probably realistically did have some ability to shut it down: He displays an inability to both set and recognize boundaries with those around him. He violated personal boundaries at the beginning of the story by filming the teacher's affair. He violated Honjou's personal boundaries by filming her fap session in the library (even though she was into it, he didn't know at the time at did it anyways). He pushes boundaries in his filming session between what is/isn't okay - Using his dog to rile up Honjou while blindfolded, forcing Sensei to piss herself while dressed as a dog, etc. He isn't innocent in all this whatsoever, even as a minor, he exhibits problematic behavior. With Honjou the boundaries between her being his actress vs. his girlfriend continue to blur, well after he put his self-imposed ironclad rules into place. And here with him hooking up with Sensei he once again fails to adhere to the boundaries he is supposed to respect, and in doing so potentially ruining his relationship with Honjou which was becoming more romantically developed.
Anyways this long of a comment this late after release probably is overkill, but I felt like coming to the author's defense on this because I'm really enjoying the story and hope that others can also try to appreciate the deeper messages rather than just remarking at the basic plot point events.
Edit: I'm also slightly concerned about the fact that homeboy apparently straight up nutted inside Sensei and she didn't seem to think twice about it..