Sorry for late reply and this lore dump.
Masuo harem is active as long as he has the ring and Ryoko is alive.
Here's how girls behave if they have sex with him. The ones who love him will refuse to have sex with anyone else such as Reiko, Aoi, Tatsuka, Ena, etc. The one he doesn't wanna let go will also not have sex with their partner but they will still love them. Examples are Junsa, Momoko (Very important for a major questline). The one he doesn't care much about will have sex with other like usual such as Tsukiko, Tomo, Sumire, Harleen.
There will be major questline in future where his ring will get destroyed, undoing everything that has happened because of ring. This would make certain NPC's fate different, some would cease to exist and Masuo will refuse sex with anyone resulting in no assistant except one. Reiko is Masuo's crush but without Ryoko's influence she doesn't reciprocate it. She will be Kunio's girlfriend (This depends on what Masuo decides to do after his ring is destroyed).
But Masuo will still have a version of Aoi to look after him as long as player doesn't play a certain map. Player will be able to correct the timeline after seeing three heart break events. Reiko & Kunio together, knowing about his sister's death and seeing Aoi dying. Because of too much stress he will fall in coma and memories of two people will guide him out of it. Now depending on who Masuo decide to side with, things will play in that direction.