Rio is from a shrine and he can speak to uka-chan, a fox deity. Nagi is from another family, but they're kinda cousin, he can talk to a crow.
Rio was supposed to marry yuri from nagi family as a family custom to maintain his power, but yuri ran away one day..
Rio was OK with it. He thought if they got married maybe he'll end up liking her which means he doesn't have feelings for her atm..
Nagi thought maybe he'll be useful, so he offered himself as a replacement.
Long story short, rio didn't want nagi as a replacement. They gradually fell in love, had sex, and rio got his power full.
Happy ending, the end. Uka is so cute.
The Crows Marriage