It's not about what humans are "meant" to do. In our culture, this is the tradition and many people happily accept it.
The issue here is you and your gf aren't on the same page, which might mean you're not with the right person. There's nothing wrong with wanting to play some more. You're only 25. Most guys I know aren't ready at your age. She's on a different track.
Also, why do you say that "naturally" she's pressing you for marriage after 2 years? Obviously, she doesn't know you well enough to realize you're not ready which, by definition, means SHE isn't ready. We're talking about the biggest step of all.
This is the issue you're facing right now. It sounds like you're overdue for one of "those" convos all couples have. It may not be fun, but the one unfair thing you could do is let her go on believing it's a possibility right now. It isn't. That doesn't mean it's impossible down the road - only you know that. But right now you're living a lie if you told total strangers something she doesn't know.