Have you tried bringing this up when she's calm and not already talking about marriage? I can see both sides but I'm more on yours since I am about your gf's age and do not plan on getting married before I'm 27 at least (been with my bf almost 3 years as well). You two need to communicate properly about this rather than arguing. It might help to look into couples counseling just to have an objective mediator to help you both get your thoughts out without it turning into a shouting match.

I think she is going about it the wrong way and just trying to annoy you into marrying her. She might be worried about fertility issues, commitment issues, or maybe her family is pressuring her. This is why it is so important to have talks where you can equally express your feelings. However, if she continues to ignore your feelings and just pester you all the time, it might be time to reconsider whether your goals line up. If she wants to get married so badly now, she might need to find someone else who also wants that. I personally find it hard to relate to her but other commenters have given good insight - the question then is whether you see yourself spending the rest of your life with someone who tries to passive-aggressively/bully you into giving in to her. I may be pessimistic here but is she going to do the same thing when she wants kids? A house? A car? A move?

So really, you need to get to the bottom of this in a calm, rational discussion (preferably with a mediator!) so you can both lay it all out, decide if any compromise is possible (long engagement?), and how to improve communication for the future if you do stay together.