Nah bro, yaoi is an outdated term used mostly to describe doujinshi. As that term had a lot of stigma in Japan it was changed to BL (boys love), which is the direct translation of "shounen ai". BL, like shounen and shoujo, is published in magazines specifically to BL.
Yaoi and shounen ai has no actual meaning regarding the amount/existence of sexuality explicit stuff in the story. Yaoi was used to describe doujinshi that are only sex without any plot, which makes sense as yaoi stands for "yama nashi, ochi nashi, imi nashi ("no climax, no point, no meaning").
Now, Koogi, the creator of Killing Stalking, said so herself that Sangwoo is straight, not in love with Yoonbum, and only sees Yoonbum as his mom. In addition, Koogi said that Killing Stalking isn't a romance and shouldn't be viewed as such.
Killing Stalking is a psychological horror. You wouldn't call a horror/drama movie porn nor romance if the story focuses on the abusive relationship between two extremely mentality ill individuals and features sex, as the story isn't advertised as neither porn or romance.
Regarding those others other abusive relationship stories that are referred to as BL, you need to realize that not only are they advertised as such, they also don't acknowledge the abusive relationship and aren't made specifically to show the horrors and the psychological aspects of one. They are usually made to fit specific fetishes and kinks of the audience and/or creator.
TLDR:
You're wrong
"Now, Koogi, the creator of Killing Stalking, said so herself that Sangwoo is straight, not in love with Yoonbum, and only sees Yoonbum as his mom. In addition, Koogi said that Killing Stalking isn't a romance and shouldn't be viewed as such."
That's just blatantly untrue. Koogi has never explicitly talked about how Sangwoo feels towards Yoonbum, beyond saying that it's "explained in the final chapter." She's otherwise allowed the character's motivations & emotions to be left up to interpretation. And in regards to whether or not KS is a BL . . . It's listed as a BL on the website it's originally published on. That alone makes any argument otherwise mute. But then on top of that, Koogi has talked explicitly about why she chose to make a story like KS be a BL. You can't say it's not a BL when the author has spoken about the fact that she purposefully created the story to be one.
"You wouldn't call a horror/drama movie porn nor romance if the story focuses on the abusive relationship between two extremely mentality ill individuals and features sex."
Many people would, actually . . . There's a whole genre basically describing that. 'Dark Romance' is precisely what you've described and is what KS is. Killing Stalking handles the horror in the story extremely well and it's often times very guttural and brutal, but many scenes between Yoonbum and Sangwoo are needlessly sexualized and by the end of the story I would say also romanticized, and the marketing team *and* Koogi knew that especially since Koogi has spoken multiple times about the fact that she would constantly draw Sangwoo to be sexy and objectified. Killing Stalking is a horror, no one's denying that, but it's not a story with one singular genre. It's also a BL. It's a dark romance story, the whole point is to be dark.
Or acts in a way that makes you think it's possible the character has it. Wanting something to read that I can relate to lmao