I went into that discussion as MacsRule suggested and I think it's fixed.
Short version: Use your Windows 8.1 retail key to register Windows 10.
Full version: I read the suggestion of using the Win 8.1 retail key, so I decided to try. I booted Windows in Fusion and opened the Activate dialog, then I entered the key. But then I had a brain fart and thought "Wait, I should be activating this booting natively" and I canceled the operation, and I got a message saying the activation had failed. It looks like even if you cancel the key is checked by Microsoft, though, because to my surprise Windows activated. I've booted through Bootcamp and through Fusion several times now and it is activated in both sides.
Thanks for the help!