Is It True that I can be Stronger if I Make Love to the Devil's Daughters?

Synopsis

Orphaned after the his parents were murdered, Uta Rain was ultimately auctioned off as a slave. Certain that he would be treated terribly no matter where he was taken, Uta was in despair. But what awaited him was... a luxurious meal and five pretty sisters. However, they were the daughters of the demon lord who was defeated by the hero 16 years ago?!