Mahou Shoujo wa Kiss Shite Kawaru 1

After Minakami Iori fell in love with Kiriya Kento at a young age, she wanted to be his wife more than anything, and Kento felt the same way about Iori. When Kento is nearly killed by a monster, she is told the only way to save him is to have sex with other men to make her more powerful as a magical girl.