Mahou Kishi Lynfolia

Kamiyama Haruka is a beloved girlfriend of protagonist Ichijou Mamoru as well as Magie Ritter Lynfolia — one of the magical knights fighting monsters and demonic beings. Bound by an unbreakable bond as battle partners, the two suffer a brutal defeat against a powerful demon lord, and Mamoru is captured as a prisoner of war.

Left behind, Haruka forms a temporary partnership with her comrade Yahagi Yajuurou to rescue her lover. But to unleash her full power as a Magie Ritter, she must resolve the "reactive lust" — a physical craving that follows combat. After much internal struggle and resistance, Haruka reluctantly gives in and gets physical with Yajuurou. There, she discovers a profound pleasure she never knew existed.

When they finally succeed in rescuing Mamoru, she realizes the terrible truth: her body has changed... and now only sex with Yajuurou can satisfy her.

[Translated from the official site]