Yeah honestly I don't think people are pissed off mainly that Siwoo and Dohyeok didn't get together, because we've had BL where the main couple broke up at the end and the backlash wasn't as big. This one was mid because it opened up a lot of things - Dohyeok being gangraped and his trauma as someone who himself did so (pressured Suha), Siwoo having some feelings for the guy who abused his closest friend, the gangrape itself - and closed none of them well.
The only thing it barely closed was that Siwoo rejected Dohyeok, which in itself is fine, but Siwoo can't suddenly take the moral high ground at the end of it all when he raped Dohyeok as well.
I knew they wouldn't end up together but this ending is way too open, like author made Siwoo into a rapist for no real development??? (He is a rapist by the way considering Dohyeok was passed out when Siwoo was putting in all those sex toys) Like Dohyeok changing??? Apologizing to Suha??? Dude got gangraped and that's never addressed?????? No therapy???????????? He just gonna wrap a scarf and get over it???? Like damn this story opens up a lot of dark themes and doesn't address them well