The sacrificial bride is actually a boy.

Seven hundred years ago, the village was on the verge of destruction due to famine. A shrine maiden's daughter made a pact with the mountain god: in exchange for offering one daughter from her lineage as a divine bride every seventy years, the village would be blessed with fertile land.

In the present day, however, no daughter was born into the shrine maiden's family. To fulfill the pact, the family raised their second son, Itsuki, as a daughter.

[Ritual of Sacrifice]
At the shrine deep in the forest, the sacrifice and other offerings are placed upon the altar.
After the marriage rite is performed, the sacrificial daughter is laid upon the altar and bound with a braided cord.
She is then left there.