Samezoku E No Sasagemono summary:

“I’m prepared to be this man’s wife...”The son of Dangata clan’s chief, Avel, is growing increasingly dissatisfied with the Miyaru clan which lives on the opposite side of the island. “The Dangata clan offers one new adult to the Miyaru clan each year.” This disgusting law was created by their ancestors in order to avoid conflict with the monstrous Miyaru clan, who are said to have the blood of sharks flowing in their veins.One day, when Avel gets attacked by a beast, he is saved by Tukiri of the Miyaru clan. Avel’s wariness is softened by Tukiri’s surprisingly kind and friendly nature, but the moment he notices Avel’s bleeding, Tukiri’s condition suddenly changes!Avel is flustered by such a different Tukiri. Yes, the Dangata clan possess bodies with a special trait where, “the first time you have sex determines your gender role.” If one plays a female’s role the first time they have sex, their body will become that of a female…!“I can’t… I’m next in line to be the chief…!”While resisting, Avel is swallowed up by a whirlpool of pleasure?