Alright, Ben, I'm gonna bring back my more innocent self to try to help you out here, and I hope what I have to say makes sense.
Given that you said she's your pastor's daughter, I'm going to assume that you're both Christians, and by the sound of it, you both seem to be pretty into following God. I'm no theology major, but I do know that the Bible says that lustful feelings (which is what your girlfriend is having and is trying to get you to help her act on) are sinful when acted on. Your girlfriend may or may not know this, but it would be best to bring this up. I know there's a part of Matthew which talks about looking at a woman lustfully, and lustful words are mentioned again in 2nd Peter 2 (I looked it up on biblegateway), but this is legitimately one of the biggest struggles adolescent Christians go through. Assuming you believe everything in the bible is 100% truth, what your girlfriend wants to do is in fact sinning, there's no way around that.
I definitely applaud you for holding to your morals in this. It really takes a good man to say no when his significant other wants to push those lines, so that's definitely commendable. I really hope this info helped you out a little bit. Good luck
~Summer