Send her to take the MSF class, and see if riding interests her (I don't recommend anyone trying to teach their S.O. to ride, for a variety of reasons, and if she DOES develop an interest in riding on her own, MSF instructors will prefer she come to class with no "bad habits.")
Sigh her up for the class, buy her some basic gear (if you haven't already), i.e. helmet, jacket and gloves (and proper footwear would be good as well), pick up "Proficient Motorcycling" for her (an excellent read) and see how she likes it.
Be warned, however, riding isn't for everyone, and if she's scared, pressured or simply not interested, you can't MAKE her ride, and you might just have to take her along as a passenger every now and then to make her happy (which, really, is that SO bad?