It would depend on the relationship and how long we were together.
If we were together for a long time and she had always led me to believe that she was straight by talking about how she thinks bisexual or lesbians are disgusting or how she could never imagine herself with another woman, then suddenly told me that she was bisexual all along... then I would feel betrayed.
If we were together for a short time and were still in the period of opening ourselves up from time to time, then I would be understanding and talk about it with her.
