This game's story (if you can even call it that) is one hell of a clusterfuck. Through MTL-poetry and like 3 different dreamstates my interpretation is as follows:
Main girl lives in a fantasy world parallel to MC, she creates some sort of dream world to get away from being a sex-slave where MC meets her in his dreams. He falls in love with her, then dreams become nightmare as main girl someone hims to another dream/world where the NTR takes place. However, main girl here doesn't remember having met MC. They train for sex fights, then loose them all. However, all the (human) opponents seem to know main girl too, often act like they were in some form of a relationship with her, yet she remembers none.
Once you get all game over scenes, MC finally grows a pair and picks up a girl that looks like main girl bar her monster girl parts. He fucks her silly and forgets all his dreams.
So, I think there's 4 worlds. MC world, FMC world, dream world, NTR world. FMC exists in all of them, but doesn't share memories between them. All the guys in the NTR-world know her either from her sex-slave world or her dream world (like MC). MC gets NTRd in his dreams, but eventually ends up with her human version, so he wins in the end.
Anyone else? No? I'm the only one that made an effort to try and understand this mess? Color me shocked...