I read this short story in a fantasy / horror? anthology in 2002, I think the story is from the 1990s.
In a foresty world there is a man who has powerful magic but the mind of a child. He (and others like him) have no conscience. They're not evil, they just do things they shouldn't and think it's funny.
In this world the people (villagers?) have a solution. At a young age they bond boys who have magic to a "mistress" who is like a dominant wife to him and serves as his conscience. She tells him what to do and he obeys her. This way society gets the gift of his magic without the destruction he would cause if he were left to his own devices.
In the story the woman he is bonded to (they have been bonded for many years and she is fond of him) finds out that she is dying. They have sex one last time and she lets him give her a fantabulous orgasm with his magic (she does not normally allow this) and then she kills him. It was poignant and sad but she had to kill him because she couldn't risk him living without her controlling him.
One other scene I remember is when she's gone for a few days and one of the villagers comes and asks him to magic something for him, and he did it but in a maliciously compliant way. I want to say it was fire ants? It was an example of the harm he could do if left unchecked.
The title of the story may have been a made up word for the female character's role - e.g. "Mistrix" (mistress + dominatrix) or something like "goodwife".
This has been bugging me for years I hope someone remembers it!