Ato 6-ika de Horobiru Gakeppuchi Kokka no Harem ni Shoukan Sarete Shimattan

I dedicate this chastity to the hero! An otherworldly harem adventure story where the protagonist, Ota, is unrivaled with his erotic skills. He is reincarnated in a doomsday kingdom that is about to be attacked by a horde of monsters and destroyed in six days. Ota has acquired the cheat skill "Emperor of Sex," which gives him a huge amount of experience points depending on his erotic experiences. He tries to avoid destruction by having sex with heroines of various professions and races, but...

(Source: Big Comics, translated)