Shin! Ryoujoku Battle Royale

Hisao, a friend who you'd never been too close with, suddenly showed up with breathless takes of his reckless foray into a world of sexual extremes. He told you of his amazing experience... Illegal fights with beautiful women in an underground arena. Getting directly involved in the battle royale. Freedom to completely violate the opponent if you win.
It sounded unbelievable. But Hisao's chiseled frame suggested he'd been fighting. And if Hisao could do it, you definitely could.

"Do you wanna try?" he recommended it to you. "It'll change your life!"

Dubiously, you accepted... and one week later you were fit and ready...

[From DLsite English]