I wouldn't say its abusive per say. Yeonwu has consented. Yeonwu has a degradation kink, not to say he wouldnt be happy to have Taewhan treat him loving. Yeonwu had a ball gag, hes into bdsm. I agree I dont think Taewhan's motives aren't great at this point but to an extent they are both enjoying this. Obvoiusly not as much as they could if they had a good relationship. I think Taewhan is still angry over the past but seeing Alex and Yeonwu bond has definitely made him miss the close relationship that they once had and also I think he's very slowly been forgiving him. Imo Taewhan was falling for Yeonwu in highschool. In the scene where he has his head on Yeonwu's shoulder Taewhan startles and apologizes. Yeonwu wondered why he apologized. I think it was because he almost kissed him. I'm betting that will come out when Taewhan reveals his past to Yeonwu. I think Taewhan will realize he had and still has feelings for him. If Yeonwu hadn't kissed him triggering his ptsd surrounding his traumatic past I think they would have eventually end up together back then. I work with domestic violence and trauma victims, although harsh Taewhan's terrible attitude and coldness is the coping mechanism he developed to protect himself. Not to make excuses for him but he was just a kid when he was potentially assaulted. The walls are breaking down, he will get there. I think in the end we will all be shocked by how much his behavior and attitude will change.
I think you might be right about the high school and I agree that Taehwan's personality is a coping mechanism. But I don't know about the rest...
To me it doesn't seems that Yeonwu has a degradation kink, it's more like he's going along with anything Taehwan wants because even angry hate sex is better than no sex ('cause he loves sex after all). Him having a gag doesn't mean he has to be into bdsm, he might have it just to try it out.
At this point I don't think Taehwan is angry that Yeonwu and Alex got close because of love jealousy, but possessive jealousy. Right now, to him Yeonwu is just a toy and he doesn't like to share...
I really hate seeing Taehwan like this. Though Yeonwu agreed, Taehwan is being abusive, taking all the anger and resentment on him. It's not healthy... But omg, the smut is good and I'm hooked since season one, there's no help. At this point I just hope Taehwan will get better...