That is explained in the conversation between Chikage and Momoki's dad. The reason why he was selected was because he was descendant of the man of the story, it was the first time that both "sacrifices" of the villages were descendants of the couple of the story and had the right age to make the ritual. That's why Chikage and Momoki's relationship is special and they get visited by their ancestors (the original couple of the story/legend).
seconding xerxesregnard, but i think they mentioned in the beginning that the village was either majority elderly or there were young children so......
They said that about Momoki’s village. That there were no other single women of age.
And there were no older men from chikage’s village. But they never said there weren’t suitable women. They’re just used to picking women from the woman’s village and men from the man’s village lol
They coulda just gotten a woman from the other village (the OG man’s village). Aka coulda just swapped genders since Momoki is already a dude, all they had to do was find a woman in the other village instead of a college kid… but bl logic