Zenryaku, Oniichan wa Seijo ni Narimashita.

What seems like an ordinary spring break evening for Umehiko and his little sister, Momo, quickly turns to confusion when a mysterious symbol appears in the sky. Next thing the siblings know, they're in a strange world that's definitely not their own. But Umehiko is only concerned with making sure his sister gets back home safely. Unfortunately, right as he sends her back into the magical seal, he's pulled away and stranded in a whole new world! Theodorus, a powerful sorcerer who's been working tirelessly to summon a blessed maiden, declares that Umehiko will become his spring for unlimited power and puts a strange symbol on the confused man's body. Suddenly, Umehiko is overcome with an uncontrollable lust—that only Theodorus can satisfy. Can he endure being the sorcerer's new "blessed maiden," or will he manage to find a way to escape?

(Source: Seven Seas Entertainment)