Fukushuu ni Moeru Yuusha-sama, TS Shite Buji Maou-sama no Oyome-san ni Naru.

When a hero set out to defeat the Demon King, the Demon King himself appeared in front of the hero and asked the hero to marry him. When the hero woke up, he had been transformed into a woman. The hero is forced to do naughty things with the demon king.

(Source: Fakku!)