The way I see it during that part, seme was alr in the midst of transforming into a yokai. It seems to be accelerated by being depressed(?). So when the uke didn't want to accept him at first, he became depressed and desperate and I believe that was the yokai side showing.
Was it necessary for the story? A lot of yaoi have unnecessary rape that easily gets brushed off, which a lot of us don't like. However, in this chapter 1, the seme was alr almost turning into a yokai and that rape scene showed that he IS dangerous if he does turn full yokai. The cat's warning would make less impact too if it weren't for that scene. Ofc none of us ever want to see rape even in fiction but sometimes I think it lends a lot to storytelling.
spoiler warning:
so? why is the rape scene in chapter 1 necessary? it could've been perfect if not for that particular scene.