18+ Renai Grimoire: Saikyou Doutei no Yuusha-sama ga Kekkon Shinai to Sekai wa Metsubou suru Sou desu

The hero had finally defeated the demon lord, sealing him. However, the seal he had used was only temporary. Should evil rise again, the descendants of the hero shall follow in his steps. At least... That was the plan. It's too bad that the hero is still a virgin! If he doesn't have children by 30, he'll become a wizard! And so, the hero desperately begins his journey to find a wife...

(Source: Square Enix, translated)