Hello and welcome to Just Answer, Washington State does not permit common law marriages. Here is a fact sheet on common law marriages, the states that do recognize them and the requirements. http://www.unmarried.org/common-law-marriage-fact-sheet.html
Info from the IRS:
You are married and living together as husband and wife.
You are living together in a common law marriage that is recognized in the state where you now live or in the state where the common law marriage began.
You are married and living apart, but not legally separated under a decree of divorce or separate maintenance.
You are separated under an interlocutory (not final) decree of divorce. For purposes of filing a joint return, you are not considered divorced.
Therefore in the state of Washington a marriage for IRS purposes would be a legal union between a man an wife.
I am very familiar with the above IRS document. I wanted to find the definition of husband and wife in Washington state law.
Yes, that is the first link.
How can I get back to the first link? Its' buried somewhere now.
Oh, I found it.
Here is the actual legal statue for the state of Washington... http://apps.leg.wa.gov/rcw/default.aspx?cite=26.04.010