Dungeon de Tatakai Tsuzukete 20-nen no Doutei Yuusha, Chijou ni Modottara Danjohi 1:1000 no Sekai Datta

The legendary hero Ryuuto, who vanished into the depths of a Dungeon to subjugate the Demon King, wandered inside for 20 years before returning to the surface. However, the world he returned to was one where the male population had plummeted due to the Demon King's curse, and women's sense of chastity had drastically changed. Every woman he meets is a beauty, and what's more, they all vie for Ryuuto's attention. While Ryuuto unintentionally charms those around him and causes commotions, he senses the shadow of the Demon King in the monsters emerging from the Dungeon and throws himself back into the battlefield. The strongest (virgin) magician's harem fantasy begins!