Waiting till marriage.
As loving and wonderful as your girlfriend may be, she needs to understand that you have boundaries, too. Negotiation sex can be a major hurdle for many couples, and there are a lot of people in your position -- you are trying to live your life by a certain batch of morals, and they're in conflict with someone you care deeply about.
If you truly believe in abstaining from sexual intercourse before marriage, then don't give up on that. If you cave into the pressure she's applying, there's a darn good chance you're going to feel really lousy about sex because of the guilt you'll feel afterwards. That's no way to start your sex life. At the same time, don't rush yourself into marriage before you're really ready just so that you can have sex (I know a few people who went this route, and financial hardships were a major issue for them).
The big thing you have to do is have a major talk with your girlfriend. Make sure she totally understands that you're serious about abstaining from sexual intercourse. Be clear about what you do feel comfortable with, and try to work with those options. Tell her that the pressure she puts on you to have sex is starting to get to you. And definitely make it clear that your choice to abstain is not her fault -- she might feel that your avoidance of sex is rooted in her, or that you find her undesirable.
Here are some things you need to read to get a perspective on the situation at hand: