I don't get the "it's just a piece of paper" argument to not getting married.
Every woman grows up picturing thier wedding day and it's my opinion that guys who use this are being totaly selfish towards what 99.9 % of the women out there want, they all want the "big day" and if it's "only a piece of paper" then what's holding you back from giving the women you love 1 day to be all about her! (and a little about you!!!
).
By saying you never want to get married you are asking her to sacrifice something she has probably wanted her entire life. Why is your preferance more important than what she may want.
Let me finish by saying that I don't know you or your gf so it may be that she really doesn't want to get married but I think it might be that she loves you and is willing to sacrifice being married to just be with you.