Seriousy dude, there's a zillion reasons aside from not getting pregnant that a girl might take it. Aside from the already mentioned benefits of a perfectly regulated, light, short cycle, no PMS, personally speaking BC also makes my skin perfectly clear and boobs half a cup bigger. It's not an indicator of anything.
I think it's risky to be having unprotected sex with her...sounds like there are some red flags. Unless you have the chat about being exclusive..good luck!