1. It's not bad having sexual thoughts with another girl (unless its violent sexual thoughts i.e. rape, necrophilia etc.)
2. I'm Christian and so are my parents but they have never told me that it's a sin to have sexual thoughts. So you're not gonna go to hell, you're parents were raised a certain way having certain beliefs, doesn't mean you have to as well.
3. Everyone's different, I never been attracted to guys, nor have I ever had a desire to see gay porn. Maybe you're not gay maybe you are, who knows. To be certain, your gonna have to actually have sex with a girl and judge from there.
My judgement...maybe your bi-curious. Based on your situation its gonna be hard to know for sure if you're straight, gay or bi. You seem like a good kid, just confused. Screw what your parents said.