You just had some amazing sex with your girlfriend, and you could tell that she was turned on even before you touched her. In the afterglow, she asks if you'd like to know what got her so hot. Obviously, you say yes. She starts by talking about how you had a fantasy of a threesome, but not with another woman. With another man. She tells you she hasn't been able to stop thinking about it. She goes in detail of her imposed bi threesome fantasy, including your mutual friend as the other man. By the end, she's turned on again.