So what I'm understanding here is the shrine girl borrows a fox youkai powers to combat monsters. But for one reason(possession?) or another she must have sex with men to revive a powerful demon. MC guy finds out at the end(he realizes his feelings) but is murder immediately and transform into a girl to be a play thing for the others. As for female MC her memory of him becomes hazy and she accepts that this is her life now.
Previous commentor is probably right about the borrowing fox spirits power thing but I'm guessing she was a shrine maiden born into this position, or who ended up there the same way the boyfriend does at the end. She seems normal but now she's like getting horny or something as indicated by the night terror before the bus incident, probably the true nature of the fox spirit power or whatever. It seems like she gradually spirals and gets worse while avoiding her bf, who in the end learns by coming to the temple and getting murdered. At the end though the arm that comes out of her is some beast, as is seen by her getting carried off and worshipping at a throne later etc. The sex was maybe to summon him or prepare her for him. After getting murdered bf turns into a girl, and also forgets his past, restarting the cycle? idk it was hot but confusing