Thank you for sharing your thoughts. i also think that senpai is dimesexual, at first i thought he was asexual but, after that volume where morinaga start working and senpai start sharing the apartment with todokoro-kun, he started having sexual dreams and was even bent up. that's when i figured he, senpai, might be dimesexual. he admitted it himself that morinaga is so special to him, but i don't think he considered him a brother since he got jealous over morinaga when masaki first appeared. after that, and in that very same volume he admitted ( although to himself ) that he actually had fallen in love with morinaga. ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄ i think from there on, senpai has started to recognize his feelings more, and instead of justifying himself that he is being " blackmailed " he realized why he really wanted morinaga beside him. although i am really excited to when morinaga also recognize senpai's feelings using straightforward words, and i know this way too far from happening now, maybe the mangaka is going to give a little push with the appearance of one of morinaga's past hook ups (≧∀≦). i am 2 years later for replying to your comment. but just like you, i finally got to write what's been on my mind for years hehe.
I have always loved the idea on making a psychological analysis on Souishi/Senpai because of how weird and unreasonable he can be when it comes to Morinaga specifically. Their relationship started normal as colleges and evolve into friendship. Morinaga saved Senpai from a teacher but at most that only made them closer as friends and helped mostly Morinaga to learn more about Souishi and his personality. Then when Morinaga first confessed while sick Souishi ignored it and pushed it aside, even making excuses and denying it happen or thinking of it as a mistake. Then during their first time he clearly wasn't into it since he is a homophonic. The next morning he pissed and disgusted but he felt comfortable enough to feel embarrassed. He threatened to kill or hurt Morinaga for what he did, but the moment he was hit with some of the blame he hesitated and at the end he just told Morinaga to get lost "for some time" Making it sound like after some time they could just go back to normal, this was confirm in a later chapter.
Later on when Morinaga left he was ridiculously upset even crying and worrying about him been dead. Then Morinaga came back and Senpai was pissed but not about what he did to him, but cause he was worried about Morinaga. During their "negotiation" and in many other occasions Senpai referred to what Morinaga was doing as blackmail, but it's not. Blackmail only works when the person doing the blackmailing has something that the other person is willing to pay for. It makes no sense for Souishi to want to keep Morinaga around after what he did to him. Much less for him to use his body to keep him around, yet he did and keep going like that. Later during different situations he said as a way to excuse himself that the only reason he could do those things was because it was with Morinaga. He couldn't do that with anyone else. And even if its not shown that often Souishi has gotten more comfortable with intimacy and would even initiate it at times. Well as much as I love to keep writing pages upon pages about this guy ill just give my conclusion. Souishi has never been a very sexual person, his sex drive is non existing most of the time so I assume he is asexual or something that makes more sense giving his situation is that he is a demisexul. He might not looked it but he is very family oriented and quite protective person. I assume that Morinaga's friendly demeanor and capabilities gained him a place within Souishi's heart even if he himself didn't realize it. This would explain why he was so desperate to keep Moringa around at the stake of his own comfort. There is more I could say and would love to but its late and I have to go to sleep if anyone wants to discus this character feel free to tell me what you think. I might elaborate some more at another time. It was fun to finally write some of this, is been in my head for years.(￣∇￣")ヾ(❀╹◡╹)ﾉ