Angmado Sunaereul Handaguyo?

Demon. Though it signifies the devil, Satan, and all evil in the world, here it's just a stable, high-paying, popular job. Woo-Hyuk, a convenience store part-timer struggling daily to pass the Incubus exam, held only once a year, one day receives a startling proposal from Si-Woon, his long-time, ever-helpful friend... “You want to pass the incubus exam that badly? Then practice with me.” A sex class begins for incubus exam prep... It feels good, but between friends... Is this okay...?! (Source: MrBlue, translated)