Ohime-sama wa Tokkun Chuu!! 2 Uketsugareshi Seinaru Majutsu

Monsters start to appear at a peaceful town. Magicians investigate the case and find out that the source of invasion is a dimensional distortion. Princesses with royal power are sent to deal with the situation. They are gifted with powers to control distortions, but need protection on the road, so main character is chosen to accompany them. Elfarina is the princess with the least magical power, so she seeks ways to be useful. She finds a book about sexual magic that raises power, but now she requires a male to undergo a full-scale study course.

[From vndbreview]