Brumak55,
You say that when your gf sits on your lap that you get an erection and drip cum - I would tend to believe that you like girls. Getting aroused when you see another penis, is not necessarily a bad thing. Many folks get aroused seeing another good looking naked body (male or female - remember there is such a thing as a man-crush) but at 13 years old I don't think you need to be worried about your sexuality or sexual preferences.
I'm a girl that can get aroused when I see an very fit and good looking woman, does that mean I'm gay? No, I can tell you that I am very into men, but I still can get moist in my crotch looking at a naked gal.

Hope this helps,
Wendy