Is it normal that my best friend turns me on...?

I'm openly bi-sexual, and I'm quite young (in my teens) and I get outrageously turned on when my best friend (who is the same gender as me; female) bites me and gets on top of me.
At first, I told her "bite me" as a sarcastic comeback,... but then she did. I now sometimes say it pretending to be sarcastic just so she will. She bites really hard too, I've gotten a bruise from it once. Sometimes when I sleepover night at her house she gets on top of me too, not just sitting on my lap either...
The only problem is, she's straight (almost to the point of homophobia) and she would never be with a girl. I've told her I was bisexual, she was one of the first people I told, and I think she knows she's turning me on and has fun with it. I don't want to ruin our friendship by admitting anything, even though I have sort of hinted it, and I'm sure she knows what she's doing... but advice would be nice.