The question might be why does she (unknowingly) keep picking guys who are not (totally) straight. I don't know what the social situation is where you live, but this seems like a bit of a pattern for her.
I wanted to ask that question, too.
At the end of the day, regardless of what her answer might be, you're not doing her any favors by intentionally suppressing your desires just because it might be too painful for her to find out that she managed to hook up with another guy who seemed straight, but wasn't entirely.
Nobody can tell you what to do; you're going to have to figure that out for yourself. Maybe it's time for you to do a mental reset? Suggest to your girlfriend that before you two get any more serious, you need to take a time out and sort through some issues. You don't even have to tell her what the issues are, she just needs to know that you're not out screwing around behind her back. You can find non-judgmental therapists who can help you explore your feelings and think about your options by going online and using Google to see who is in your area.
A couple of thoughts:
- No serious mental health professionals think we're capable of changing our preferences so don't look for therapists who can help you change or cure you. Instead, focus on therapists who state they have experience helping clients work through understanding their sexual preferences and its implications.
- Your happiness and mental health should, and must be, your primary focus. If you are happy, you will have happy relationships. You may feel pressured by guilt, by shame, or by familial influences to do something that you know is not going to make you happy. Resist those!
After you feel comfortable with your discussions with your therapist and the results and decisions you've come to, you can sit down with your girlfriend and explain how you feel. Honesty is the absolute best policy here. You may ask her to stay with you and she might agree to that -- however, begin with the end in mind: you're making decisions about
your life, not about a particular relationship.
Good luck,
stevenotsean.