hmmm, something sounds fishy to me
Why don't what you propose is a MFF with her included. That way if she chooses to expore her bi curious part of her, she can. If she doesn't want to, then she'll still get to see you doing it with another woman. Maybe just explain to her that you don't really want to just go out and have sex with another woman without including her in on the fun.
I'd be afraid that she's testing you by letting you go out and play with another woman. KWIM????