I don't exactly understand your question but if you're asking about the reason behind those events then here it goes:
After meeting that guy that attacked him in highschool, Aki was reminded of "how wrong it is to be gay" and that he is dragging Saku down the wrong road so he wanted to have sex one last time to remember it, probably, then he broke up with him and then Saku left
because to he can't control his emotions and himself whenever he's with Aki. To him, he has crossed the boundary and would never able to go back like siblings anymore. That's why he left home. It's better for him to live apart from Aki rather than living with him and pretending that nothing ever happened between them.
Can someone please tell me why whenever Aki said he just wanna be brothers after having sex with Saku and Saku would leave home? I was just wondering~