She didn't fuck anything moving because of the spell, she fucked anything moving because she's part Fairy and they love sex.
They are into free love and sharing sex around.
The spell was stopping her fucking the MC. It was to keep the keymaster away from the spell unlocking the keystones full potential so the spell she had placed on her repulsed her from him.
She cockblocked him because she wanted him and got jealous others could have him but she couldn't.
She didn't lie about anything else, in fact she's pretty upfront about who and what she is. The only thing she doesn't tell him is why she can't sleep with him and that's because she doesn't know. It's Mallory that figures out she's had a spell put on her.
Sorry for my bad writing ; yep I knew all that, I meant only the part with not sleeping with MC was caused by the spell. I'm ok with her being a fairy who love to sleep around.
Plus, if i'm right, it's said at one point that sex is more or less a kind of payment or a way to thank people in this world, much less "sacred" and serious than in ours.
But she lied to MC and to her fairy friend just to cockblock them and have MC for herself even if she was disgust by him. So sex is free and sharing but not for him cause she wants him but doesn't want him... I know it's the spell doing the "not want him" but if she was a nice girl, she'd accept the sharing like she accept it for everyone else.
But thanks for your explanation ; we can always count on big sis for help and constructive criticism (and I'm saying it unironically)