Kagachisama to miko by Sano Fuyuko and illustrated by Takashina Yuh is a totally conventional yet hilarious entry into the isolated location with a weird tradition that includes a kind of "arranged marriage" genre. In this novel there is a small island that is blessed with relatively good weather and abundant harvests from the land and the sea. This is thanks to the protection of the sea god. The sea god selects a priestess and a...err...stand-in by marking them in specific spots. They are partnered for life, and when one dies the other also dies. The priestess and the stand-in go to an even smaller adjacent island for a three-day "ceremony" (which turns out to mostly be sex between the priestess and the stand-in who is possessed by the sea god). In this story the main character, Mizuya, is chosen as the "priestess" (first guy chosen ever!) and the guy he'd left the island to forget is chosen as the stand-in. The stand-in, Kaiji, is the son of a previous priestess, the only child ever born to a priestess, and is an albino. He was feared as a godly child and grew up in relative isolation. Mizuya, much younger than Kaiji, had met Kaiji as a little child. He hadn't been afraid of Kaiji and they became friends. But when Kaiji becomes betrothed to Mizuya's sister Mizuya is filled with jealousy and realizes he loves Kaiji. He is filled with guilt that he's resenting his sister and can't fathom his feelings for Kaiji (he's never heard of a guy in love with a guy--his innocence is adorable), so he asks to go away to work. His sister never marries Kaiji as she's chosen as a priestess. He works hard on the mainland, trying desperately to forget his feelings. But then he has to return when he learns of his sister's death. He also finds the mark on his body and agonizes whether he step up and become the "priestess." In the end he finds that he can't put the island in jeopardy. He is even more shocked to find that Kaiji is the stand-in.
Basically Mizuya then leads a life of isolation. No one wants to be around the priestess because they don't want to offend the god. Also, Kaiji is rather possessive. There is some forced sexual contact, though no penetration with a penis (because that'd be sex! and the priestess has to be a virgin for the sea god!). The forced sexual contact gets worse after Mizuya is nearly assaulted by another islander, including lots of random non-penis things being shoved in Mizuya's ass. Mizuya is in the painful situation of having intimate contact with the person he loves but knowing that the contact is out of duty and not out of love. (oh, the agony!)
In the end, Mizuya is "deflowered" by the sea god possessing Kaiji (after some "tentacle rape" with sea plants instead of tentacles). Kaiji doesn't want to put Mizuya through the experience again but Mizuya does not want to abandon the island. Kaiji wants to figure out a way to end the contract. The islander who had nearly assaulted Mizuya had snuck onto the island with Mizuya and manages to break the contract. The rich family of the island, of whom Kaiji is a member, will have to support the island as they transition to normal conditions with the immense wealth they had accumulated over the years.
They learn that the contract between the island and the sea god had originated because of a girl's wish to bring her dead brother back, whom she loved more than as a brother. She had promised that her brother's descendants would submit to the ceremony as well. Ever since, girls burning with a forbidden love would be chosen as priestesses and the men they loved would be chosen as the stand-ins. They were allowed to have an officially sanctioned affair (the stand-in would be married to some other woman). This fact reveals that Mizuya loves Kaiji. Naturally this leads to Kaiji revealing his own love for Mizuya (he doesn't want to say just how long he's been hiding his own feelings for Mizuya--probably because he's probably wanted Mizuya from when Mizuya was a child), and they finally become a real couple.
Yeah, this story isn't very good. I skimmed some bits. But it was still amusing to me. I was particularly amused by the fact that Mizuya had thought Kaiji was in love with Mizuya's sister the
entire time until Kaiji explains that he'd wanted to marry Mizuya's sister to bind Mizuya to him...Which caused Mizuya to run away instead. Good thing the sea god didn't discriminate by gender and chose Mizuya as priestess!