How can I keep from getting an erection when spending time with my girlfriend?

I'm 16 and just had my girlfriend over for dinner about 8 hours ago. We were watching a movie when my parents went to sleep and she started to cuddle with me. And when she started rubbing my chest I almost lost it. We're not sexual, and both have abstinence pledges, but I'm still a guy with those urges. She laid her head on my lap and felt the "zipper bump" that happens when you sit while wearing jeans, and she thought it was my penis. So she sat up quickly and seemed freaked out. I explained that it was just the zipper, but I'm scared of what would have happened if she actually felt my *****. Please help. "and if your a girl, would you be creeped out, or interested?"