Not just in this song ^^, but in the real world too.
Better than “wife.” Better than “I haven’t looked at the issue, but I sure wish Arizona recognized common law marriage.” [Ed.: it doesn’t, and if it had, you didn’t qualify anyway.] Because “girlfriend” means no commitments or promises you did not explicitly undertake. And no 64-day waiting period, when
the irreversible happens it’s time to make a change.
Better than “friends” on the internet. Because “girlfriend” means you have presumably met IRL at least once.