Wash. Rev. Code § 26.04.260
If two persons in Washington have a legal union, other than a marriage, that:
(3) Does not meet the definition of domestic partnership in RCW 26.60.030, then they shall be treated as having the same rights and responsibilities as married spouses in this state, unless:
[ 2012 c 3 s 11 (Referendum Measure No. 74, approved November 6, 2012).]
Notice—2012 c 3: See note following RCW 26.04.010.