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.
Aphrodisiac aside, I did love the classic genderbend rendition of having the guy (gently) pushing the neo-girl's boundaries until she drowns in the pleasure of being fucked as a girl. And even better, the first intercourse didn't magically overwrite her almost tsundere reluctance, letting me enjoy it twice in one book.
I would have enjoyed it even more if the hero's girl form had been more petite, and there was a bit of a missed opportunity to show an internal cumshot and impregnation (since the story just a page later reveals that she does carry his child). But even without that, this is a good genderbend story.