Man, there are layers to the horror in this work.
The protagonist himself doesn't seem to believe in the village legend (which is in line with his profession as a doctor and his scientific education), but he does love the village and its people. Unlike some works where the protagonist and heroine would give up everything to be together with each other, the protagonist here does love the heroine, but not enough to give up his life in the village and join her in the city instead.
Seiji!! You've got a great career, a handsome face AND the childhood friend advantage! Why didn't you try to pursue her normally instead of tricking her into the ritual? The heroine doesn't seem to have any romantic feelings for him at all at first...
The protagonist keeps reminiscing fondly about his childhood with the heroine, but his dialogue makes it clear that she's only left the village for 3 years (and been working in the city during that period of time). So, even accounting for the time the protagonist spent away at university in the city too (as a doctor, he's probably a little older than the heroine then, to account for his residency too), they must've spent some time together as grown adults too.
The fact that he doesn't seem to have any recent memories worth remembering with the heroine makes me think that they've actually been growing apart (at least on her end)? Which aligns with her being shocked when he confesses his feelings to her.
The play in the work itself is (relatively) vanilla, but that in itself is scary in the context of the story. There's something about the contrast between the lovey-dovey (unilateral) acts and the context of the ritual...
At the end of the work, it seems that the heroine has become at least a little mentally shaken, which even Seiji acknowledges at some points :( I'm definitely thinking too much about this, but how will she take care of their child in that mental state, while facing the villagers who trapped her in this situation in the first place daily? I'm going to be imagining an alternate ending where she escapes with her child, or where Seiji finally realizes how unhealthy the village will be for the heroine and their child and leaves...
There are a surprising amount of hidden (sub)plotlines that belie the duration of the plot tracks too... Like the implication that Seiji has been stalking the heroine extensively while she was in the city.