While not the main plot, the Mexican classic Pedro Páramo contains a fragment that haunts scholars: the character , a mule-driver (burrero), is driven to murder out of a distorted love for his donkey, Prudencia . In Rulfo’s elliptical prose, Abundio confesses that after his wife died, Prudencia became “the only soft breath I knew at night.” When a drunken man insults the donkey, Abundio kills him with a rock.
This is a niche but culturally significant intersection of genres and tropes. While "romantic storyline" implies a human-to-human relationship, the "donkey" often serves as a rather than a romantic partner. However, if you are looking for texts where the relationship with the donkey is the core emotional driver (platonic life-partners) or where a donkey facilitates a human romance, here are the most useful categories and specific texts. Men Sex With Donkey
Sexual agency requires a level of communication and mutual understanding that does not exist across species. While not the main plot, the Mexican classic
And she does. In her own way.