I'm not your lawyer and this is not legal advice, etc.
IF YOU STAY IN WASHINGTON, WATCH OUT, BRO.
A 2-second google search showed me that Washington state recognizes some sort of nonsense called , which means that if you and your girlfriend live together and split expenses, she may have a claim on all your property, even if it is 100% owned by you and even if you do not hold yourselves out as being married.
To avoid this, you will need to enter into a cohabitation agreement with your girlfriend, clearly setting out what is yours and what is hers (for example, all real estate now owned or later acquired).
I'm not licensed to practice law in Washington state. You need to find someone who is.
IF YOU MOVE TO TEXAS, things are a lot better. Texas is a community property state, which means that any property you acquire before marriage is your separate property. Your girlfriend wouldn't acquire anything by living with you, paying bills, etc.
Now, Texas does recognize common-law marriage, but honestly it is extremely rare. To have a common law marriage recognized by a Texas court, you have to (1) agree to be married, (2) live together as husband and wife, and (3) tell everyone ("hold yourselves out") you are married. Here's a good summary of Texas law on common law marriage.
Good luck.