Saw the raws. I dont know japanese but...(spoiler)... looks like the seme was also abused by his mother(?). And then the seme and uke are licking each others wound. But to be honest, I still dont know why the seme has to rape the uke. It is a terrible plot by the mangaka to make them have sex with each other. To be honest, the premise is good, but it was executed poorly.
Boku o Warui Ko ni Shite