*whimpers at the sight of reading lesbianism stories*
Let's this get out of the way but...its sexy...really sexy honestly
But back on the topic, the best best way to approach this is
TALK to her. Alot of thing can go wrong if you or your friend just ignore it and that will affeck your feelings as well hers. She might think that you're rejecting her and visa vera.
You're going through a normal stage in your life where people at your age are starting to think more about sexuality preferences. Am I straight? Do I prefer people of the same sex. Some questions that aren't answered can be answered by experimenting and then looking back at your experience.
Socially, we're at a stage where homosexuality are becoming more acceptable IMHO and that paves the way to allow the concept to be more open. You seem to be open-minded about it but your friend isn't, perhaps she hasn't accepted homosexuality or she's afraid of the social rejection if she accepts it. I can only spectulate it but the next time you're caught in the situation with your friend where there's an awkward moment of silence or its hard to talk about that particular topic. Take a chance! Talk! Be open about it and when its over, you and your friend will absolutely be relieved and will thank me one day
I wouldn't be too surprised at the amount of people that response yes to the question "Have you fiddled with homosexuality?" on this forum.