Well, this is my own interpretation of course but I think it's close enough (I'm a literature major xD)
So, Yashiro has gone through a lot of sexual trauma since he was a kid, and he has managed to cope with his trauma by internalizing it as "who he is". He has internalized ideas of him being a slut, liking pain, needing that to get off or he doesn't feel anything etc... That sexual encounter he had with Doumeki shook him to the core because it shattered all of these notions. It made him realize his trauma and the unfairness of what he went through, the child rape and subsequent rape ( He realized it when he was cumming over and over again Despite Doumeki's gentle and loving treatment. He kept begging him to be rough because he couldn't handle the pleasure alone). As for why he left him, I saw two reasons: 1.being with Doumeki entails him denying everything he believes about himself, starting over, facing his trauma, healing, etc.. I think he couldn't get himself to make such a drastic change overnight and felt that Doumeki didn't even understand his struggle. 2. When Doumeki almost died saving him, he received a second shock. "What if he dies for me?". This is where his internalized self-loathing probably intervened to tell him that it's better to let Doumeki go and be free. In his head, he gave him a chance to go back to a safe civilian life, away from "the filth" that he considers his life.
i would really like if someone could explain yashiro’s behavior to me. when he and doumeki did it and he cried, why did he cry? also what was the reason for throwing doumeki away? i’m not good with philosophical themes so if anyone has any interpretations share pls