Admittedly, it starts off as a mistake. Maybe not for him, but she would never sleep with a man that she met in a bar, especially when it was their second time meeting. But, there's something about him that she finds that she likes, truly honestly likes, and what's the harm in keeping this... this thing between them going? It's not like it'll ever be serious.
She's half of a person and he shouldn't exist, but somehow they manage to fit. Two jagged pieces completing the puzzle that is their life.
Or, how The Evil Queen and Robin of Locksley came to be.