Somebody who likes who regardless of your differences is likely to be a friend you can count on, and one who won't ditch you over superficial stuff. If you have been friends with this girl for a long time even though you disagree about important things, that's a valuable friendship to have, and worth some trouble to keep.
I don't know if you realize it, but the way you tell it, you presented your orientation as a decision based on logic, and not as a recognition of your true feelings. You did NOT tell her that you had no choice and this is who you are. Since you presented it as a CHOICE you had decided on after thinking about it logically, you shouldn't be surprised if she gave you arguments on the other side that she thought you might not have considered.
Clearly, and understandably, you are in a state of turmoil right now. But, while I can see how her arguments would hurt you, I also see that you were disrespectful of your friend's beliefs.
Look, it is reasonable for you to expect your friends to accept you for who you are, and respect you. That's kind of a requirement for friendship. But it's not reasonable to demand that your friends all change their ideas and think exactly the way you do, or even approve of everything that you do.
If you want to try to hang on to that friendship, I suggest that you tell her you haven't "decided" to be bisexual; you have discovered that you are. And whether it makes sense to her or you or anybody at all, it's just how you feel and you're going to have to live with it. (I'm sure you would like it to be all neat and logical, but feelings are not logical and you shouldn't try to explain them as such.)
