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It was superpowers more than magic, but the anime Mai-Hime had a contrived explanation for virgin only powers. Powers come from nanomachines in the blood. They only work in females. But females whose body have contacted sperm develop antibodies that destroy the nano-machines.

So they only work for virgin females, for a particular definition of virgin. It was intentionally set up this way by higher-tech people back when because women are supposedly more trustworthy or less violent or something. And you don't want them using their powers to set up hereditary dynasties. Yeah there are holes in that, but that's what the story says.

(Yeah, kinda creepy, like a lot of anime if you starting thinking about it.)