Well because at the end of the day, you'll need someone besides your family that knows you best, and at the same time, be able to bump uglies all you want. I don't know whether you do it now or not, but coming from a society that believes in sex after marriage, that would be the reason.
I think both of you really need to sit down and talk this out, and maybe you have thought of this: are both of you financially stable; if one of you couldn't be financially stable, could that other person step up to the plate; can you tolerate every bad habit that goes on within the relationship; are you two capable of providing for each other emotionally and spiritually, etc..
Also, you feel like you're going to be trapped? In what way? That you can't mess with another girl if you chose to? Other than that, I don't see how else you would be trapped. If that's something hard to handle, it's justified, if you're with her long enough eventually you would want to marry.
And as far as she is concerned, is she being realistic about marriage, if not, then she too needs to think within those guidelines above, and possibly more.
-Anonymous