My girlfriend of 3.5 years told me that she "can't marry me"

I didn't propose, but we have been talking about it for a little while now. She doesn't want a family/kids or anything and I do. She also says she doesn't really trust me with some things.

She says she still loves me and we are still together. I don't have any interest in any other women and have few friends so I'm not really in any position to dump her. Plus her family are very good people and I enjoy their company.

I'm planning on leaving where I currently live in approx 18 months anyways. What do?