Throughout this, something seemed off, and it wasn't until I read Baniita's comment that I was able to pinpoint what exactly: first, it's that Reiji is presented as more and more feminine; and second, it's that Kasai isn't involved enough with Reiji from a romantic standpoint, to convince me that this is a relationship that will last.
Reiji compares himself to a "wife", then that other guy says he "moans like a girl", then Reiji fawns over Kasai like a girl from a shoujo manga... It's not like male characters can only be gruff testosterone beasts, so it's distasteful to compare them to girls if they're anything but that. That's one writing choice I dislike in yaoi: that's a man, so why are you comparing him to a woman?
This ties into the other complaint, about Kasai not looking invested in having a romantic relationship with Reiji, who's doing all the heavy lifting and taking the "pester him until he falls in love" approach. Kasai comes across as aro-ace — he's uninterested in pursuing either on his own, and has to be pushed into it because Plot. Even in the extra, two years into their living together, Kasai isn't initiating anything unless he's drunk, which is...just sad, man. In the end, he seems to just go along with what Reiji wants; he would've been perfectly content to remain a mentor figure to him, and was downright uncomfortable with all of Reiji's advances. But this is a yaoi, so they have to get together and have sex because Plot.
It's not a downright bad story. The art is good, and I did like the revenge porn subplot and how Kasai cared about his students. But it really, really needed more active, enthusiastic participation from Kasai, and less shoujofication of Reiji, to make the relationship credible.