18+ Toushin Toshi III

Toushin Tournament -- an annual event held in the Toushin Toushi. All contestants are required to register with a beautiful female partner, and the winner of each battle gains the privilege of doing anything they want with the loser's partner (except harming them). The final winner of the tournament earns the title of Toushin, and gets to live freely in the Toushin District of the city with his partner.

Five years ago, Nakuto's father participated in the Toushin Tournament, saying he'll be back when he wins. That was the last Nakuto ever heard of him. Now, having grown up and hoping to discover the fate of his father, Nakuto sets out to enter the tournament his father disappeared in.

(Source: AliceSoft Wikia)