If two persons in Washington have a legal union, other than a marriage, that:
(1) Was validly formed in another state or jurisdiction;
(2) Provides substantially the same rights, benefits, and responsibilities as a marriage; and
(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:
(a) Such relationship is prohibited by RCW 26.04.020 (1)(a) or (2); or
(b) They become permanent residents of Washington state and do not enter into a marriage within one year after becoming permanent residents.