I'm kind of in the same boat as you. Sort of, except my boyfriend is the one with the plans..and I'm the one steering clear of them. I'm 17 (18 in a few months) and he's just turning 16 (we've been together for almost 8 months..have been good friends for years though). Is your girlfriend the same age as you? I think at the time in your life it's really important to focus on getting your shit together, i.e career, school, ect...not really marriage and children. I know how you feel, you're probably just as scared as I am at the fact that you have to GROW UP..and marriage is just a step too far?

Honestly, I would bluntly tell her.."Listen babe, this isn't about you. I'm not going to break up with you, and I plan on having you in my future, but at this point in my life, I don't want to think about marriage or children. While I do want those things, I just don't want them yet. Give me a few years to become financially stable so when I do want those things, it's not a spur of the moment thing and we won't be broke or overly stressed"..

something along those lines? I don't really know what else to tell you if you've already told her straight up what the deal is.

I don't think I helped much, but I feel you, bro. It'll work out..if you two really love eachother and shit is as good as you say it is, then this should work out. Just be honest and sensitive.