Erotical Wizard to 12-nin no Hanayome

Ichinose Junpei was about to drop out of Tokyo's magic academy because he couldn't demonstrate any magical aptitude. Out of nowhere a beautiful witch named Meryl appeared in front of him and said, "I need your power, as the reincarnation of the Demon Lord." Not interested in becoming the new Demon Lord, Junpei shares his secret with Sonia, a beautiful girl who is the descendant of the original Hero, and asks her to help him channel his power properly. But as it turned out, the demon lord's magical power is based on sex. This is the start of a story about battling with a harem of beautiful girls with his "Ero Magic"!!

(Source: HJ Bunko, translated)