Shirasawa, being 16 years old, is over the age of consent in Japan. He is legal. Plus, Ekoda did ask him what he wants to do. If not for the alcohol involvement, this would be perfectly legal.
Didn't one mangaka got in trouble for having sex with 16yo prostitutes? And in manga, in Boy's Abyss teacher who fucked her 17yo student the MC got insta-fired and ostracized by her neighbors, and on police watchlist, so clearly it's not considered fine and cool in Japan. Age-gap couple in manga often wait until the younger one is adult, like in Tawawa on Monday where the JK girlfriend shows up on her birthday "I'm 18 now, take my virginity sensei" to make it look better morally, which also shows clear and enthusiastic consent from the younger one , while older one is often apprehensive at first, also to make it look better. Here it was anything but that, she did everything to reduce his ability to consent, and in fact he's regretful next morning, as you mentioned before. I don't think forbidden age-gap must always be wrong in fiction, but author need to show me it's as consensual as it gets and they usually try to do just that.
Alcohol is normally totally forbidden for minors, any moral adult in manga will always prevent youngster from drinking. And total LOL at Galder for bullshitting calculations, in manga minors can get drunk on liquor chocos or even fumes, if minor is shown behaving like drunk then he's drunk. In josei manga and manhwa, especially horny ones, MC sometimes ends drunk in bed with hot guy in first chapter, sometimes even sober one, but she's always adult twenty-something or older and usually the one to initiate the sex and basically drunkenly sexually assault that hunk. I wonder if it's done to make the main girl look not like a slut for having casual sex, by making it a "mistake". Again, sex with drunk minor, no matter how slightly, is something I've seen always avoided in any manga or manhwa that's not rape hentai, even having fever from those three-four fatal drops of rain is sign to cancel sex.
In this case there was several simultaneous factors that made this as un-consensual as possible without it being outright forceful rape. Again, sensei did everything to make him, minor, way younger than her, virgin, and first time dealing with being drunk, as unable to freely decide to consent to sex as she could given the circumstances. Given that last chapter author make the joke of sister being dumb enough to not notice her coworkers cum on her at work, and that coyness with the non-canon rape side-story, I'm not willing to trust author that he thought that through.