Sengoku Youko Kitan: Mononoke Miko to Takeda no Shinobi

Tenzo, a ninja in peril, was saved by a fox maiden, Chiyome. To repay her, who has been living in eternal solitude, Tenzo takes Chiyome's virginity and becomes a half-demon himself, so he can take her as his wife. Demon and youkai find their way in marriage, a love story set in the Warring States period!

(Source: Bishoujo Bunko, translated)