Someone recently asked me: "Domestic partnerships are almost the same as marriage, why the need for marriage?"Under California State law, a heterosexual couple may also enter into a Domestic Partnership. So, try this: Inform your girlfriend that you've decided you'd like to enter into a Domestic Partnership with her, instead of marriage. I am certain that she will be able to explain the HUGE difference between the two realities in a far more forceful and colorful way than, I would ever be able to accomplish.
Just one friendly word of advice from someone who used to play hockey in High School, you should put on an athletic supporter with a protective cup before having the aforementioned conversation with your girlfriend.