18+ Toushin Toshi III

The protagonist Nakuto, has his sights on becoming the "War God" aka the Champion of a gladiator style tournament with a twist. When Nakuto tries to enter this tournament, he learns that there's a special condition required for entering. He must partner with a beautiful woman along as collateral, and so by chance he finds his childhood friend, Hazumi in town worried for him. Since he has no other choice he enters Hazumi as his partner. Much to Nakuto and Harumi's horror they learn of the reason for the female partners. Winners of these tournament matches can do whatever they so please to the losing opponent's partner for a whole day.

(Source: MU)