This story hasn't really been subtle about the two of them dealing with their isolation due to the "abnormal" feelings they have--that is, being gay. I think it seems likely that while said monster is a real physical entity in the story, the author means it as a representation of the experience of being gay IRL, where you feel like there's something wrong with you and keeping you away from everyone else.
Am I wrong? It seems like a pretty obvious interpretation to me.
i dont think the monster really representing anything with the girl.
Like it saved them and sent the girl flying once, or even the village is scare of the monster. if it homophobia that not work. And Kon even go to the cave to play.
Now i think of another idea after read what you said. The monster is the idea of gay itself. The people treat it as bad, and even the girls not sure of so they also scare of it, because that is "sick". but they could also find their true self, hence Kon actually even go play at the cave.
Still, even that idea also have conflict. Like how even outdated people treated gay as something to be punish rather than scare of and offer sacrifice.
and even as the girl find each other, was happy and embrace they "abnormal" somehow they wake up at home don't know what just happen and where the other at. not sure what that representing