魔王さまは男の娘!（Maou-sama wa Otoko no Ko!）

"I've found you, HEROOO!"
"Yes?"
"Take responsibility for what you've done to me!"
"What's this, now?"

In the middle of my college studies, the words abruptly crashed in. I listened to this girlishly dressed boy yell his problems at me. He (she?) was a crossdresser from a parallel world, apparently. The evil ruler of that world, apparently. But the hero from that world destroyed his (her?) castle, apparently.

"Wait wait wait, you're saying I'm that hero?" I said. "Look. I have NO legendary bloodline, I had NO dormant powers waiting to be unleashed by fate. I had no dead lover or whatever, to give me strength to fight. I'm a normal guy."

"No you're not! And on top of destroying my castle, you did THIS to me!"

When the parallel world hero defeated the girlishly dressed boy, he cursed him (her?) to into being male, and to never grow older... apparently.

"You made it so I can never be anyone's bride! Take responsibility!"
"....... Sure."
"....... Huh?"

And from that one-way exchange, my sex life with a crossdressed ex-girl demon king began.

[From DLsite English]