Densetsu no Yuusha to Saiyaku no Maou no Ren'ai Jijou ~Yuusha wa Maou no Haha ni, Maou wa Yuusha no Mama ni Koi o Shita~

On his journey, the hero met a beautiful woman with a sorrowful expression. Meanwhile, the demon king, while burning down a village, fell head over heels for a spirited woman at first sight. The hero fell in love with the demon king’s mother. The demon king was captivated by the hero’s mother. The two women they loved soon realized who the other was. Though guilt toward their sons gnawed at them, they couldn’t turn away from the pleasure and love they found. And so, they didn't stop, even as they knew the men they loved were destined to kill each other… [Translated from dlsite]